Date: Fri, 25 Jan 2013 15:54:28 -0500
From: Bruce Momjian <bruce@momjian.us>
To: Tianyin Xu <tixu@cs.ucsd.edu>
Cc: pgsql-docs@postgresql.org
Subject: Re: Manual Entries of commit_delay and commit_siblings are
	not complete.
Message-ID: <20130125205428.GQ6848@momjian.us>
References: 
 <CABBDWweXsOJiMOBTsJ3K4Cs0E8J8YKQS3hWk6_3GR2UFOykPGA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="qoTlaiD+Y2fIM3Ll"
Content-Disposition: inline
In-Reply-To: 
 <CABBDWweXsOJiMOBTsJ3K4Cs0E8J8YKQS3hWk6_3GR2UFOykPGA@mail.gmail.com>
User-Agent: Mutt/1.5.20 (2009-06-14)
Precedence: bulk
Sender: pgsql-docs-owner@postgresql.org


--qoTlaiD+Y2fIM3Ll
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline

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. +

--qoTlaiD+Y2fIM3Ll
Content-Type: text/x-diff; charset=us-ascii
Content-Disposition: attachment; filename="fsync.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>

--qoTlaiD+Y2fIM3Ll
Content-Type: text/plain
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
MIME-Version: 1.0


-- 
Sent via pgsql-docs mailing list (pgsql-docs@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-docs

--qoTlaiD+Y2fIM3Ll--