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Subject: Manual Entries of commit_delay and commit_siblings are not complete.
From: Tianyin Xu <tixu@cs.ucsd.edu>
To: pgsql-docs@postgresql.org
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--14dae934044dc9349304cd144d46
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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

--14dae934044dc9349304cd144d46
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<div class=3D"gmail_quote">Hi, pgsql-docs,<div><br></div><div>I was confuse=
d by the system behavior of PG&#39;s write ahead log, specifically the rela=
tion between=A0<span style=3D"text-align:left;font-size:12px;font-family:mo=
nospace">commit_delay</span>,=A0<span style=3D"text-align:left;font-size:12=
px;font-family:monospace">commit_siblings</span>, and=A0<span style=3D"text=
-align:left;font-size:12px;font-family:monospace">fsync</span>.</div>

<div><br></div><div>Basically according to the manual (<a href=3D"http://ww=
w.postgresql.org/docs/9.1/static/runtime-config-wal.html" target=3D"_blank"=
>http://www.postgresql.org/docs/9.1/static/runtime-config-wal.html</a>), I =
know:</div>
<div>
<br></div><div><span style=3D"text-align:left;font-size:12px;font-family:mo=
nospace">&quot;commit_delay&quot; </span>specifies=A0<span style=3D"line-he=
ight:18px;text-align:left;font-size:12px;font-family:verdana,sans-serif">th=
e delay before a transaction attempts to flush the WAL buffer out to disk.<=
/span></div>

<div><span style=3D"text-align:left;font-size:12px;font-family:monospace">&=
quot;commit_siblings&quot;=A0</span><span style=3D"line-height:18px;text-al=
ign:left;font-size:12px;font-family:verdana,sans-serif">specifies m</span><=
span style=3D"line-height:18px;text-align:left;font-size:12px;font-family:v=
erdana,sans-serif">inimum number of concurrent open transactions to require=
 before performing the=A0</span><tt style=3D"line-height:18px;text-align:le=
ft;font-size:12px">commit_delay</tt><span style=3D"line-height:18px;text-al=
ign:left;font-size:12px;font-family:verdana,sans-serif">=A0delay.</span></d=
iv>

<div><br></div><div style=3D"text-align:left">However, after looking at the=
 code, I figured out these configuration parameters are also controlled by=
=A0<span style=3D"font-size:12px;font-family:monospace">fsync</span>.</div>
<div style=3D"text-align:left"><br></div><div style=3D"text-align:left">The=
 only usage of the two parameter I can find is:</div><div style=3D"text-ali=
gn:left"><br></div><div style=3D"text-align:left">/*=A0src/backend/access/t=
ransam/xact.c=A0*/</div>

<div style=3D"text-align:left"><br></div><div style=3D"text-align:left"><di=
v>static TransactionId</div><div>RecordTransactionCommit(void)</div><div>=
=A0 =A0 =A0...</div></div><div style=3D"text-align:left"><div>=A0 =A0 =A0if=
 (CommitDelay &gt; 0 &amp;&amp;=A0</div>

<div>=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 enableFsync &amp;&amp;</div><div>=
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 MinimumActiveBackends(CommitSiblings))<=
/div><div>=A0 =A0 =A0 =A0 =A0 =A0 pg_usleep(CommitDelay);</div><div><br></d=
iv></div><div style=3D"text-align:left">=A0 =A0 ...</div>

<div style=3D"text-align:left">}</div><div style=3D"text-align:left"><br></=
div><div style=3D"text-align:left">where, CommitDelay stores the parameter =
of &quot;<span style=3D"font-size:12px;font-family:monospace">commit_delay<=
/span>&quot;, CommitSiblings stores the parameter of &quot;<span style=3D"f=
ont-size:12px;font-family:monospace">commit_siblings</span>&quot;, and=A0en=
ableFsync=A0stores the parameter of &quot;<span style=3D"font-size:12px;fon=
t-family:monospace">fsync</span>&quot;.</div>

<div style=3D"text-align:left"><br></div><div style=3D"text-align:left">It =
seems the two parameters (<span style=3D"font-size:12px;font-family:monospa=
ce">commit_delay</span>=A0and=A0<span style=3D"font-size:12px;font-family:m=
onospace">commit_siblings</span>) only have effect when=A0<span style=3D"fo=
nt-size:12px;font-family:monospace">fsync</span>=A0is set to be *on*.</div>

<div style=3D"text-align:left"><br></div><div style=3D"text-align:left">But=
, unfortunately, such behavior is not covered in the manual. The mistake I =
made is only set the two values but disabling=A0<span style=3D"font-size:12=
px;font-family:monospace">fsync=A0</span>so the expected PG does not behave=
 as I expected. I think it&#39;s definitely helpful to specify the relation=
s between these configuration parameters either in the manual, or in the so=
urce code by giving warnings logs.</div>

<div style=3D"text-align:left"><br></div><div style=3D"text-align:left">Tha=
nks a lot!</div><span class=3D"HOEnZb"><font color=3D"#888888"><div style=
=3D"text-align:left">Tianyin</div></font></span></div>

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