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[nested] [flat]
* Quoting in stored procedures
@ 1999-08-28 21:27 [email protected]
1999-08-29 00:54 ` Re: [HACKERS] Quoting in stored procedures Tom Lane <[email protected]>
0 siblings, 1 reply; 266+ messages in thread
From: [email protected] @ 1999-08-28 21:27 UTC (permalink / raw)
To: pgsql-hackers
The syntax needed for defining functions is quite annoying because of the
needed quoting. Example:
CREATE FUNCTION bla (int4) RETURNS char AS '
BEGIN
RETURN ''Two quotes needed''
END;
' LANGUAGE 'plpgsql';
How about allowing something like in shell here-documents:
CREATE FUNCTION bla (int4) RETURNS char LANGUAGE 'plpgsql' UNTIL '__EOF__';
BEGIN
RETURN 'Only one quote needed'
END;
__EOF__
Or similar. The __EOF__ is of course totally arbitrary and every string can
be used, so this will work with any language used.
jochen
^ permalink raw reply [nested|flat] 266+ messages in thread
* Re: [HACKERS] Quoting in stored procedures
1999-08-28 21:27 Quoting in stored procedures [email protected]
@ 1999-08-29 00:54 ` Tom Lane <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Tom Lane @ 1999-08-29 00:54 UTC (permalink / raw)
To: pgsql-hackers
[email protected] writes:
> The syntax needed for defining functions is quite annoying because of the
> needed quoting.
Actually, considering that you probably don't want to be typing (and
retyping) procedure definitions right into psql, it seems to me that
what's really wanted is a procedure editor. Once you've got that,
it could mask annoying little details like doubling quotes.
Another thing that I'd like to see along these same lines is an "UPDATE
FUNCTION" command that allows the stored text for a PL function
definition to be replaced, without dropping/recreating the function
definition. Drop/recreate changes the function OID, thereby breaking
triggers that use it, and is generally a real pain when you're trying
to debug a bit of plpgsql...
regards, tom lane
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20 Pavel Borisov <[email protected]>
0 siblings, 0 replies; 266+ messages in thread
From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)
Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
1 file changed, 128 insertions(+)
create mode 100644 src/backend/access/heap/README.XID64
diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. (1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base. (2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. (3)
+XMAX = t_xmax + t_xid_base/t_multi_base. (4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
--
2.24.3 (Apple Git-128)
--cpok4wp6gsarlzvp--
^ permalink raw reply [nested|flat] 266+ messages in thread
end of thread, other threads:[~2022-01-10 19:20 UTC | newest]
Thread overview: 266+ messages (download: mbox mbox.gz follow: Atom feed)
-- links below jump to the message on this page --
1999-08-28 21:27 Quoting in stored procedures [email protected]
1999-08-29 00:54 ` Tom Lane <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
2022-01-10 19:20 [PATCH v6] README for 64bit xid Pavel Borisov <[email protected]>
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