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* Manual Entries of commit_delay and commit_siblings are not complete.
@ 2012-10-03 00:36 Tianyin Xu <[email protected]>
2012-10-28 01:16 ` Manual Entries of commit_delay and commit_siblings are not complete. Tianyin Xu <[email protected]>
2013-01-25 20:54 ` Re: Manual Entries of commit_delay and commit_siblings are not complete. Bruce Momjian <[email protected]>
0 siblings, 2 replies; 3+ messages in thread
From: Tianyin Xu @ 2012-10-03 00:36 UTC (permalink / raw)
To: pgsql-docs
Hi, pgsql-docs,
I was confused by the system behavior of PG's write ahead log, specifically
the relation between commit_delay, commit_siblings, and fsync.
Basically according to the manual (
http://www.postgresql.org/docs/9.1/static/runtime-config-wal.html), I know:
"commit_delay" specifies the delay before a transaction attempts to flush
the WAL buffer out to disk.
"commit_siblings" specifies minimum number of concurrent open transactions
to require before performing the commit_delay delay.
However, after looking at the code, I figured out these configuration
parameters are also controlled by fsync.
The only usage of the two parameter I can find is:
/* src/backend/access/transam/xact.c */
static TransactionId
RecordTransactionCommit(void)
...
if (CommitDelay > 0 &&
enableFsync &&
MinimumActiveBackends(CommitSiblings))
pg_usleep(CommitDelay);
...
}
where, CommitDelay stores the parameter of "commit_delay", CommitSiblings
stores the parameter of "commit_siblings", and enableFsync stores the
parameter of "fsync".
It seems the two parameters (commit_delay and commit_siblings) only have
effect when fsync is set to be *on*.
But, unfortunately, such behavior is not covered in the manual. The mistake
I made is only set the two values but disabling fsync so the expected PG
does not behave as I expected. I think it's definitely helpful to specify
the relations between these configuration parameters either in the manual,
or in the source code by giving warnings logs.
Thanks a lot!
Tianyin
--
Tianyin XU,
http://cseweb.ucsd.edu/~tixu/
^ permalink raw reply [nested|flat] 3+ messages in thread
* Manual Entries of commit_delay and commit_siblings are not complete.
2012-10-03 00:36 Manual Entries of commit_delay and commit_siblings are not complete. Tianyin Xu <[email protected]>
@ 2012-10-28 01:16 ` Tianyin Xu <[email protected]>
1 sibling, 0 replies; 3+ messages in thread
From: Tianyin Xu @ 2012-10-28 01:16 UTC (permalink / raw)
To: pgsql-docs
Hi, pgsql-docs,
I was confused by the system behavior of PG's write ahead log, specifically
the relation between commit_delay, commit_siblings, and fsync.
Basically according to the manual (
http://www.postgresql.org/docs/9.1/static/runtime-config-wal.html), I know:
"commit_delay" specifies the delay before a transaction attempts to flush
the WAL buffer out to disk.
"commit_siblings" specifies minimum number of concurrent open transactions
to require before performing the commit_delay delay.
However, after looking at the code, I figured out these configuration
parameters are also controlled by fsync.
The only usage of the two parameter I can find is:
/* src/backend/access/transam/xact.c */
static TransactionId
RecordTransactionCommit(void)
...
if (CommitDelay > 0 &&
enableFsync &&
MinimumActiveBackends(CommitSiblings))
pg_usleep(CommitDelay);
...
}
where, CommitDelay stores the parameter of "commit_delay", CommitSiblings
stores the parameter of "commit_siblings", and enableFsync stores the
parameter of "fsync".
It seems the two parameters (commit_delay and commit_siblings) only have
effect when fsync is set to be *on*.
But, unfortunately, such behavior is not covered in the manual. The mistake
I made is only set the two values but disabling fsync so the expected PG
does not behave as I expected. I think it's definitely helpful to specify
the relations between these configuration parameters either in the manual,
or in the source code by giving warnings logs.
Thanks a lot!
Tianyin
^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Manual Entries of commit_delay and commit_siblings are not complete.
2012-10-03 00:36 Manual Entries of commit_delay and commit_siblings are not complete. Tianyin Xu <[email protected]>
@ 2013-01-25 20:54 ` Bruce Momjian <[email protected]>
1 sibling, 0 replies; 3+ messages in thread
From: Bruce Momjian @ 2013-01-25 20:54 UTC (permalink / raw)
To: Tianyin Xu <[email protected]>; +Cc: pgsql-docs
On Tue, Oct 2, 2012 at 05:36:28PM -0700, Tianyin Xu wrote:
> Hi, pgsql-docs,
>
> I was confused by the system behavior of PG's write ahead log, specifically the
> relation between commit_delay, commit_siblings, and fsync.
>
> Basically according to the manual (http://www.postgresql.org/docs/9.1/static/
> runtime-config-wal.html), I know:
>
> "commit_delay" specifies the delay before a transaction attempts to flush the
> WAL buffer out to disk.
> "commit_siblings" specifies minimum number of concurrent open transactions to
> require before performing the commit_delay delay.
>
> However, after looking at the code, I figured out these configuration
> parameters are also controlled by fsync.
>
> The only usage of the two parameter I can find is:
>
> /* src/backend/access/transam/xact.c */
>
> static TransactionId
> RecordTransactionCommit(void)
> ...
> if (CommitDelay > 0 &&
> enableFsync &&
> MinimumActiveBackends(CommitSiblings))
> pg_usleep(CommitDelay);
>
> ...
> }
>
> where, CommitDelay stores the parameter of "commit_delay", CommitSiblings
> stores the parameter of "commit_siblings", and enableFsync stores the parameter
> of "fsync".
>
> It seems the two parameters (commit_delay and commit_siblings) only have effect
> when fsync is set to be *on*.
>
> But, unfortunately, such behavior is not covered in the manual. The mistake I
> made is only set the two values but disabling fsync so the expected PG does not
> behave as I expected. I think it's definitely helpful to specify the relations
> between these configuration parameters either in the manual, or in the source
> code by giving warnings logs.
Docs updated for PG 9.3 with the attached patch.
--
Bruce Momjian <[email protected]> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ It's impossible for everything to be true. +
--
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Attachments:
[text/x-diff] fsync.diff (929B, 2-fsync.diff)
download | inline diff:
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
new file mode 100644
index 5f47c1f..19f513e
*** a/doc/src/sgml/config.sgml
--- b/doc/src/sgml/config.sgml
*************** include 'filename'
*** 1968,1974 ****
the first process that becomes ready to flush waits for the configured
interval, while subsequent processes wait only until the leader
completes the flush. The default <varname>commit_delay</> is zero
! (no delay).
</para>
</listitem>
</varlistentry>
--- 1968,1974 ----
the first process that becomes ready to flush waits for the configured
interval, while subsequent processes wait only until the leader
completes the flush. The default <varname>commit_delay</> is zero
! (no delay), and only honored if <varname>fsync</varname> is enabled.
</para>
</listitem>
</varlistentry>
^ permalink raw reply [nested|flat] 3+ messages in thread
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2012-10-03 00:36 Manual Entries of commit_delay and commit_siblings are not complete. Tianyin Xu <[email protected]>
2012-10-28 01:16 ` Tianyin Xu <[email protected]>
2013-01-25 20:54 ` Bruce Momjian <[email protected]>
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