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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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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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Hi, pgsql-docs,<div><br></div><div>I was confused by the system behavior of=
 PG&#39;s write ahead log, specifically the relation between=A0<span style=
=3D"background-color:rgb(255,255,255);font-family:monospace;font-size:12px;=
text-align:left">commit_delay</span>,=A0<span style=3D"background-color:rgb=
(255,255,255);font-family:monospace;font-size:12px;text-align:left">commit_=
siblings</span>, and=A0<span style=3D"font-family:monospace;font-size:12px;=
text-align:left;background-color:rgb(255,255,255)">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">http://www.postgr=
esql.org/docs/9.1/static/runtime-config-wal.html</a>), I know:</div><div>
<br></div><div><span style=3D"background-color:rgb(255,255,255);font-family=
:monospace;font-size:12px;text-align:left">&quot;commit_delay&quot; </span>=
specifies=A0<span style=3D"background-color:rgb(255,255,255);font-family:ve=
rdana,sans-serif;font-size:12px;line-height:18px;text-align:left">the delay=
 before a transaction attempts to flush the WAL buffer out to disk.</span><=
/div>
<div><span style=3D"background-color:rgb(255,255,255);font-family:monospace=
;font-size:12px;text-align:left">&quot;commit_siblings&quot;=A0</span><span=
 style=3D"background-color:rgb(255,255,255);font-family:verdana,sans-serif;=
font-size:12px;line-height:18px;text-align:left">specifies m</span><span st=
yle=3D"font-family:verdana,sans-serif;font-size:12px;line-height:18px;text-=
align:left;background-color:rgb(255,255,255)">inimum number of concurrent o=
pen transactions to require before performing the=A0</span><tt class=3D"VAR=
NAME" style=3D"font-size:12px;line-height:18px;text-align:left;background-c=
olor:rgb(255,255,255)">commit_delay</tt><span style=3D"font-family:verdana,=
sans-serif;font-size:12px;line-height:18px;text-align:left;background-color=
:rgb(255,255,255)">=A0delay.</span></div>
<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"background-color:rgb(255,255,255);font-family:monospace;f=
ont-size:12px">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"background-color:rgb(255,255,255);font-family:monos=
pace;font-size:12px">commit_delay</span>&quot;, CommitSiblings stores the p=
arameter of &quot;<span style=3D"background-color:rgb(255,255,255);font-fam=
ily:monospace;font-size:12px">commit_siblings</span>&quot;, and=A0enableFsy=
nc=A0stores the parameter of &quot;<span style=3D"background-color:rgb(255,=
255,255);font-family:monospace;font-size:12px">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"background-color:rgb(255,255,255);=
font-family:monospace;font-size:12px">commit_delay</span>=A0and=A0<span sty=
le=3D"background-color:rgb(255,255,255);font-family:monospace;font-size:12p=
x">commit_siblings</span>) only have effect when=A0<span style=3D"backgroun=
d-color:rgb(255,255,255);font-family:monospace;font-size:12px">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"background-c=
olor:rgb(255,255,255);font-family:monospace;font-size:12px">fsync=A0</span>=
so the expected PG does not behave as I expected. I think it&#39;s definite=
ly helpful to specify the relations between these configuration parameters =
either in the manual, or in the source 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><div style=3D"text-align:left">Tianyin</div><div><br></div>=
<div><br clear=3D"all"><div><br></div>-- <br><span style=3D"border-collapse=
:separate;color:rgb(0,0,0);font-family:&#39;Times New Roman&#39;;font-style=
:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-h=
eight:normal;text-align:-webkit-auto;text-indent:0px;text-transform:none;wh=
ite-space:normal;word-spacing:0px;font-size:medium"><span style=3D"color:rg=
b(102,102,102);font-family:arial;font-size:small">Tianyin XU,<br>
<a href=3D"http://cseweb.ucsd.edu/%7Etixu/" target=3D"_blank">http://cseweb=
.ucsd.edu/~tixu/</a></span></span><br>
</div>

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