agora inbox for [email protected]  

help / color / mirror / Atom feed

Re: [GENERAL] Point in Time Recovery WAS: Hot Backup
268+ messages / 4 participants
[nested] [flat]

* Re: [GENERAL] Point in Time Recovery WAS: Hot Backup
@ 2002-10-09 16:46  Sandeep Chadha <[email protected]>
  0 siblings, 1 reply; 268+ messages in thread

From: Sandeep Chadha @ 2002-10-09 16:46 UTC (permalink / raw)
  To: scott.marlowe <[email protected]>; +Cc: Tom Lane <[email protected]>; pgsql-hackers

I'd have agree on most of what you said. I still think most crashes occur due to data corruption which can only be recovered by using a good backup. 

Anyways my problem is I have a 5 gig database. I run a cron job every hour which runs pg_dump which takes over 30 minutes to run and degrades the db performance. I was hoping for something which can solve my problem and then I don't have to take backup every hour. Is there a plan on implementing incremental backup technique for pg_dump or Is it going to be same for next one or two releases.

Thanks much For you time 
Sandeep. 

-----Original Message-----
From: scott.marlowe [mailto:[email protected]]
Sent: Wednesday, October 09, 2002 12:19 PM
To: Sandeep Chadha
Cc: Tom Lane; [email protected]; pgsql-general
Subject: [GENERAL] Point in Time Recovery WAS: Hot Backup 


Hi Sandeep.  What you were calling Hot Backup is really called Point in 
Time Recovery (PITR).  Hot Backup means making a complete backup of the 
database while it is running, something Postgresql has supported for a 
very long time.

On Mon, 7 Oct 2002, Sandeep Chadha wrote:

> Hello to all the Doers of Postgres!!!
> 
> Last time I went through forums, people spoke highly about 7.3 and its 
> capability to do hot backups. My problem is if the database goes down 
> and I lose my main data store, then I will lose all transactions back 
> to the time I did the pg_dump.

Let's make it clear that this kind of failure is EXTREMELY rare on real 
database servers since they almost ALL run their data sets on RAID arrays.  
While it is possible to lost >1 drive at the same time and all your 
database, it is probably more likely to have a bad memory chip corrupt 
your data silently, or a bad query delete data it shouldn't.

That said, there IS work ongoing to provide this facility for Postgresql, 
but I would much rather have work done on making large complex queries run 
faster, or fix the little issues with foreign keys cause deadlocks.

> Other databases (i e Oracle) solves this by retaining their archive 
> logs in some physically separate storage. So, when you lose your data, 
> you can restore the data from back-up, and then apply your archive log, 
> and avoid losing any committed transactions. 
> 
> > Postgresql has been lacking this all along. I've installed postgres 
> 7.3b2 and still don't see any archive's flushed to any other place. 
> Please let me know how is hot backup procedure implemented in current 
> 7.3 beta(2) release.

Again, you'll get better response to your questions if you call it "point 
in time recovery" or pitr.  Hot backup is the wrong word, and something 
Postgresql DOES have.

It also supports WALs, which stands for Write ahead logs.  These files 
store what the database is about to do before it does it.  Should the 
database crash with transactions pending, the server will come back up and 
process the pending transactions that are in the WAL files, ensuring the 
integrity of your database.

Point in Time recovery is very nice, but it's the last step in many to 
ensure a stable, coherent database, and will probably be in 7.4 or 
somewhere around there.  If you're running in a RAID array, then the loss 
of your datastore should be a very remote possibility.




^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* Re: [GENERAL] Point in Time Recovery WAS: Hot Backup
@ 2002-10-09 17:11  Rod Taylor <[email protected]>
  parent: Sandeep Chadha <[email protected]>
  0 siblings, 1 reply; 268+ messages in thread

From: Rod Taylor @ 2002-10-09 17:11 UTC (permalink / raw)
  To: Sandeep Chadha <[email protected]>; +Cc: scott.marlowe <[email protected]>; Tom Lane <[email protected]>; pgsql-hackers

On Wed, 2002-10-09 at 12:46, Sandeep Chadha wrote:
> I'd have agree on most of what you said. I still think most crashes occur due to data corruption which can only be recovered by using a good backup. 
> 
> Anyways my problem is I have a 5 gig database. I run a cron job every hour which runs pg_dump which takes over 30 minutes to run and degrades the db performance. I was hoping for something which can solve my problem and then I don't have to take backup every hour. Is there a plan on implementing incremental backup technique for pg_dump or Is it going to be same for next one or two releases.
> 
> Thanks much For you time 

Oh, if thats your problem then use asynchronous replication instead. It
doesn't remove the slow time, but will distribute the slowness across
every transaction rather than all at once (via creation of replication
logs).  Things won't degrade much during the snapshot transfer itself,
as there isn't very much work involved (differences only).

Now periodically backup the secondary box.  Needs diskspace, but not
very much power otherwise.

#!/bin/sh
while(true)
do
	asynchreplicate.sh
	pg_dumpall > "`date`.bak"
done



> -----Original Message-----
> From: scott.marlowe [mailto:[email protected]]
> Sent: Wednesday, October 09, 2002 12:19 PM
> To: Sandeep Chadha
> Cc: Tom Lane; [email protected]; pgsql-general
> Subject: [GENERAL] Point in Time Recovery WAS: Hot Backup 
> 
> 
> Hi Sandeep.  What you were calling Hot Backup is really called Point in 
> Time Recovery (PITR).  Hot Backup means making a complete backup of the 
> database while it is running, something Postgresql has supported for a 
> very long time.
> 
> On Mon, 7 Oct 2002, Sandeep Chadha wrote:
> 
> > Hello to all the Doers of Postgres!!!
> > 
> > Last time I went through forums, people spoke highly about 7.3 and its 
> > capability to do hot backups. My problem is if the database goes down 
> > and I lose my main data store, then I will lose all transactions back 
> > to the time I did the pg_dump.
> 
> Let's make it clear that this kind of failure is EXTREMELY rare on real 
> database servers since they almost ALL run their data sets on RAID arrays.  
> While it is possible to lost >1 drive at the same time and all your 
> database, it is probably more likely to have a bad memory chip corrupt 
> your data silently, or a bad query delete data it shouldn't.
> 
> That said, there IS work ongoing to provide this facility for Postgresql, 
> but I would much rather have work done on making large complex queries run 
> faster, or fix the little issues with foreign keys cause deadlocks.
> 
> > Other databases (i e Oracle) solves this by retaining their archive 
> > logs in some physically separate storage. So, when you lose your data, 
> > you can restore the data from back-up, and then apply your archive log, 
> > and avoid losing any committed transactions. 
> > 
> > > Postgresql has been lacking this all along. I've installed postgres 
> > 7.3b2 and still don't see any archive's flushed to any other place. 
> > Please let me know how is hot backup procedure implemented in current 
> > 7.3 beta(2) release.
> 
> Again, you'll get better response to your questions if you call it "point 
> in time recovery" or pitr.  Hot backup is the wrong word, and something 
> Postgresql DOES have.
> 
> It also supports WALs, which stands for Write ahead logs.  These files 
> store what the database is about to do before it does it.  Should the 
> database crash with transactions pending, the server will come back up and 
> process the pending transactions that are in the WAL files, ensuring the 
> integrity of your database.
> 
> Point in Time recovery is very nice, but it's the last step in many to 
> ensure a stable, coherent database, and will probably be in 7.4 or 
> somewhere around there.  If you're running in a RAID array, then the loss 
> of your datastore should be a very remote possibility.
> 
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 3: if posting/reading through Usenet, please send an appropriate
> subscribe-nomail command to [email protected] so that your
> message can get through to the mailing list cleanly
> 
-- 
  Rod Taylor




^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* Re: [GENERAL] Point in Time Recovery WAS: Hot Backup
@ 2002-10-09 18:04  Justin Clift <[email protected]>
  parent: Rod Taylor <[email protected]>
  0 siblings, 1 reply; 268+ messages in thread

From: Justin Clift @ 2002-10-09 18:04 UTC (permalink / raw)
  To: Rod Taylor <[email protected]>; +Cc: Sandeep Chadha <[email protected]>; scott.marlowe <[email protected]>; Tom Lane <[email protected]>; pgsql-hackers

Rod Taylor wrote:
> 
<snip>
> Oh, if thats your problem then use asynchronous replication instead.

For specific info, the contrib/rserv package does master->slave
asynchronous replication as Rod is suggesting.  From memory it was
having troubles working with PostgreSQL 7.2.x, but someone recently
submitted patches that make it work.

There's a HOW-TO guide that a community member wrote on setting up rserv
with PostgreSQL 7.0.3, although it should be practically identical for
PostgreSQL 7.2.x (when rserv is patched to make it work).

http://techdocs.postgresql.org/techdocs/settinguprserv.php

That could be the basis for your async replication solution.

Hope that helps.

:-)

Regards and best wishes,

Justin Clift


> It doesn't remove the slow time, but will distribute the slowness across
> every transaction rather than all at once (via creation of replication
> logs).  Things won't degrade much during the snapshot transfer itself,
> as there isn't very much work involved (differences only).
> 
> Now periodically backup the secondary box.  Needs diskspace, but not
> very much power otherwise.
> 
> #!/bin/sh
> while(true)
> do
>         asynchreplicate.sh
>         pg_dumpall > "`date`.bak"
> done
> 
> > -----Original Message-----
> > From: scott.marlowe [mailto:[email protected]]
> > Sent: Wednesday, October 09, 2002 12:19 PM
> > To: Sandeep Chadha
> > Cc: Tom Lane; [email protected]; pgsql-general
> > Subject: [GENERAL] Point in Time Recovery WAS: Hot Backup
> >
> >
> > Hi Sandeep.  What you were calling Hot Backup is really called Point in
> > Time Recovery (PITR).  Hot Backup means making a complete backup of the
> > database while it is running, something Postgresql has supported for a
> > very long time.
> >
> > On Mon, 7 Oct 2002, Sandeep Chadha wrote:
> >
> > > Hello to all the Doers of Postgres!!!
> > >
> > > Last time I went through forums, people spoke highly about 7.3 and its
> > > capability to do hot backups. My problem is if the database goes down
> > > and I lose my main data store, then I will lose all transactions back
> > > to the time I did the pg_dump.
> >
> > Let's make it clear that this kind of failure is EXTREMELY rare on real
> > database servers since they almost ALL run their data sets on RAID arrays.
> > While it is possible to lost >1 drive at the same time and all your
> > database, it is probably more likely to have a bad memory chip corrupt
> > your data silently, or a bad query delete data it shouldn't.
> >
> > That said, there IS work ongoing to provide this facility for Postgresql,
> > but I would much rather have work done on making large complex queries run
> > faster, or fix the little issues with foreign keys cause deadlocks.
> >
> > > Other databases (i e Oracle) solves this by retaining their archive
> > > logs in some physically separate storage. So, when you lose your data,
> > > you can restore the data from back-up, and then apply your archive log,
> > > and avoid losing any committed transactions.
> > >
> > > > Postgresql has been lacking this all along. I've installed postgres
> > > 7.3b2 and still don't see any archive's flushed to any other place.
> > > Please let me know how is hot backup procedure implemented in current
> > > 7.3 beta(2) release.
> >
> > Again, you'll get better response to your questions if you call it "point
> > in time recovery" or pitr.  Hot backup is the wrong word, and something
> > Postgresql DOES have.
> >
> > It also supports WALs, which stands for Write ahead logs.  These files
> > store what the database is about to do before it does it.  Should the
> > database crash with transactions pending, the server will come back up and
> > process the pending transactions that are in the WAL files, ensuring the
> > integrity of your database.
> >
> > Point in Time recovery is very nice, but it's the last step in many to
> > ensure a stable, coherent database, and will probably be in 7.4 or
> > somewhere around there.  If you're running in a RAID array, then the loss
> > of your datastore should be a very remote possibility.
> >
> >
> > ---------------------------(end of broadcast)---------------------------
> > TIP 3: if posting/reading through Usenet, please send an appropriate
> > subscribe-nomail command to [email protected] so that your
> > message can get through to the mailing list cleanly
> >
> --
>   Rod Taylor
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 3: if posting/reading through Usenet, please send an appropriate
> subscribe-nomail command to [email protected] so that your
> message can get through to the mailing list cleanly

-- 
"My grandfather once told me that there are two kinds of people: those
who work and those who take the credit. He told me to try to be in the
first group; there was less competition there."
   - Indira Gandhi



^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* Re: [GENERAL] Point in Time Recovery WAS: Hot Backup
@ 2002-10-09 18:13  Rod Taylor <[email protected]>
  parent: Justin Clift <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Rod Taylor @ 2002-10-09 18:13 UTC (permalink / raw)
  To: Justin Clift <[email protected]>; +Cc: pgsql-hackers

On Wed, 2002-10-09 at 14:04, Justin Clift wrote:
> Rod Taylor wrote:
> > 
> <snip>
> > Oh, if thats your problem then use asynchronous replication instead.
> 
> For specific info, the contrib/rserv package does master->slave

Thanks.  I was having a heck of a time remembering what it was called or
even where the DBA found it.

-- 
  Rod Taylor




^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread

* [PATCH v6] README for 64bit xid
@ 2022-01-10 19:20  Pavel Borisov <[email protected]>
  0 siblings, 0 replies; 268+ messages in thread

From: Pavel Borisov @ 2022-01-10 19:20 UTC (permalink / raw)

Authors:
- Pavel Borisov <[email protected]>
- Maxim Orlov <[email protected]>
- Yura Sokolov <[email protected]> <[email protected]>
---
 src/backend/access/heap/README.XID64 | 128 +++++++++++++++++++++++++++
 1 file changed, 128 insertions(+)
 create mode 100644 src/backend/access/heap/README.XID64

diff --git a/src/backend/access/heap/README.XID64 b/src/backend/access/heap/README.XID64
new file mode 100644
index 00000000000..457ba9b9ef5
--- /dev/null
+++ b/src/backend/access/heap/README.XID64
@@ -0,0 +1,128 @@
+src/backend/access/heap/README.XID64
+
+64-bit Transaction ID's (XID)
+=============================
+
+A limited number (N = 2^32) of XID's required to do vacuum freeze to prevent
+wraparound every N/2 transactions. This causes performance degradation due
+to the need to exclusively lock tables while being vacuumed. In each
+wraparound cycle, SLRU buffers are also being cut.
+
+With 64-bit XID's wraparound is effectively postponed to a very distant
+future. Even in highly loaded systems that had 2^32 transactions per day
+it will take huge 2^31 days before the first enforced "vacuum to prevent
+wraparound"). Buffers cutting and routine vacuum are not enforced, and DBA
+can plan them independently at the time with the least system load and least
+critical for database performance. Also, it can be done less frequently
+(several times a year vs every several days) on systems with transaction rates
+similar to those mentioned above.
+
+On-disk tuple and page format
+-----------------------------
+
+On-disk tuple format remains unchanged. 32-bit t_xmin and t_xmax store the
+lower parts of 64-bit XMIN and XMAX values. Each heap page has additional
+64-bit pd_xid_base and pd_multi_base which are common for all tuples on a page.
+They are placed into a pd_special area - 16 bytes in the end of a heap page.
+Actual XMIN/XMAX for a tuple are calculated upon reading a tuple from a page
+as follows:
+
+XMIN = t_xmin + pd_xid_base. 					(1)
+XMAX = t_xmax + pd_xid_base/pd_multi_base.		(2)
+
+"Double XMAX" page format
+---------------------------------
+
+At first read of a heap page after pg_upgrade from 32-bit XID PostgreSQL
+version pd_special area with a size of 16 bytes should be added to a page.
+Though a page may not have space for this. Then it can be converted to a
+temporary format called "double XMAX".
+
+All tuples after pg-upgrade would necessarily have xmin = FrozenTransactionId.
+So we don't need tuple header t_xmin field and we reuse t_xmin to store higher
+32 bits of its XMAX.
+
+Double XMAX format is only for full pages that don't have 16 bytes for
+pd_special. So it neither has a�place for a single tuple. Insert and HOT update
+for double XMAX pages is impossible and not supported. We can only read or
+delete tuples from it.
+
+When we are able to prune page double XMAX it will be converted from it to
+general 64-bit XID page format with all operations on its tuples supported.
+
+In-memory tuple format
+----------------------
+
+In-memory tuple representation consists of two parts:
+- HeapTupleHeader from disk page (contains all heap tuple contents, not only
+header)
+- HeapTuple with additional in-memory fields
+
+HeapTuple for each tuple in memory stores t_xid_base/t_multi_base - a copies of
+page's pd_xid_base/pd_multi_base. With tuple's 32-bit t_xmin and t_xmax from
+HeapTupleHeader they are used to calculate actual 64-bit XMIN and XMAX:
+
+XMIN = t_xmin + t_xid_base. 					(3)
+XMAX = t_xmax + t_xid_base/t_multi_base.		(4)
+
+The downside of this is that we can not use tuple's XMIN and XMAX right away.
+We often need to re-read t_xmin and t_xmax - which could actually be pointers
+into a page in shared buffers and therefore they could be updated by any other
+backend.
+
+Update/delete with 64-bit XIDs and 32-bit t_xmin/t_xmax
+--------------------------------------------------------------
+
+When we try to delete/update a tuple, we check that XMAX for a page fits (2).
+I.e. that t_xmax will not be over MaxShortTransactionId relative to
+pd_xid_base/pd_multi_base of a its page.
+
+If the current XID doesn't fit a range
+(pd_xid_base, pd_xid_base + MaxShortTransactionId) (5):
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base/pd_multi_base on
+a page and update all t_xmin/t_xmax of the other tuples on the page to
+correspond new pd_xid_base/pd_multi_base.
+
+- If it was impossible, it will try to prune and freeze tuples on a page.
+
+- If this is unsuccessful it will throw an error. Normally this is very
+unlikely but if there is a very old living transaction with an age of around
+2^32 this can arise. Basically, this is a behavior similar to one during the
+vacuum to prevent wraparound when XID was 32-bit. Dba should take care and
+avoid very-long-living transactions with an age close to 2^32. So long-living
+transactions often they are most likely defunct.
+
+Insert with 64-bit XIDs and 32-bit t_xmin/t_xmax
+------------------------------------------------
+
+On insert we check if current XID fits a range (5). Otherwise:
+
+- heap_page_prepare_for_xid() will try to increase pd_xid_base for t_xmin will
+not be over MaxShortTransactionId.
+
+- If it is impossible, then it will try to prune and freeze tuples on a page.
+
+Known issue: if pd_xid_base could not be shifted to accommodate a tuple being
+inserted due to a very long-running transaction, we just throw an error. We
+neither try to insert a�tuple into another page nor mark the current page as
+full. So, in this (unlikely) case we will get regular insert errors on the next
+tries to insert to the page 'locked' by this very long-running transaction.
+
+Upgrade from 32-bit XID versions
+--------------------------------
+
+pg_upgrade doesn't change pages format itself. It is done lazily after.
+
+1. At first heap page read, tuples on a page are repacked to free 16 bytes
+at the end of a page, possibly freeing space from dead tuples.
+
+2A. 16 bytes of pd_special is added if there is a place for it
+
+2B. Page is converted to "Double XMAX" format if there is no place for
+pd_special
+
+3. If a page is in double XMAX format after its first read, and vacuum (or
+micro-vacuum at select query) could prune some tuples and free space for
+pd_special, prune_page will add pd_special and convert page from double XMAX
+to general 64-bit XID page format.
-- 
2.24.3 (Apple Git-128)


--cpok4wp6gsarlzvp--





^ permalink  raw  reply  [nested|flat] 268+ messages in thread