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From: Tianyin Xu <[email protected]>
To: [email protected]
Subject: Manual Entries of commit_delay and commit_siblings are not complete.
Date: Tue, 2 Oct 2012 17:36:28 -0700
Message-ID: <CABBDWweXsOJiMOBTsJ3K4Cs0E8J8YKQS3hWk6_3GR2UFOykPGA@mail.gmail.com> (raw)

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/

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