Обсуждение: Manual Entries of commit_delay and commit_siblings are not complete.

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Manual Entries of commit_delay and commit_siblings are not complete.

От
Tianyin Xu
Дата:
Hi, pgsql-docs,

I was confused by the system behavior of PG's write ahead log, specifically the relation between commit_delaycommit_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

Manual Entries of commit_delay and commit_siblings are not complete.

От
Tianyin Xu
Дата:
Hi, pgsql-docs,

I was confused by the system behavior of PG's write ahead log, specifically the relation between commit_delaycommit_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

Re: Manual Entries of commit_delay and commit_siblings are not complete.

От
Bruce Momjian
Дата:
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  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +

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