From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | "Peter Childs" <peterachilds(at)gmail(dot)com> |
Cc: | "Postgresql Performance" <pgsql-performance(at)postgresql(dot)org> |
Subject: | Re: Commit takes a long time. |
Date: | 2008-01-03 16:35:01 |
Message-ID: | 6663.1199378101@sss.pgh.pa.us |
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Lists: | pgsql-performance |
"Peter Childs" <peterachilds(at)gmail(dot)com> writes:
> Using Postgresql 8.1.10 every so often I get a transaction that takes a
> while to commit.
> I log everything that takes over 500ms and quite reguallly it says things
> like
> 707.036 ms statement: COMMIT
AFAIK there are only two likely explanations for that:
1. You have a lot of deferred triggers that have to run at COMMIT time.
2. The disk system gets so bottlenecked that fsync'ing the commit record
takes a long time.
If it's #2 you could probably correlate the problem with spikes in I/O
activity as seen in iostat or vmstat.
If it is a disk usage spike then I would make the further guess that
what causes it might be a Postgres checkpoint. You might be able to
dampen the spike a bit by playing with the checkpoint parameters, but
the only real fix will be 8.3's spread-out-checkpoints feature.
regards, tom lane
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