Re: Commit takes a long time. - Mailing list pgsql-performance

From Peter Childs
Subject Re: Commit takes a long time.
Date
Msg-id a2de01dd0801040046y7f23468aueb914f5f7ccb359f@mail.gmail.com
Whole thread Raw
In response to Re: Commit takes a long time.  (Tom Lane <tgl@sss.pgh.pa.us>)
List pgsql-performance


On 03/01/2008, Tom Lane <tgl@sss.pgh.pa.us> wrote:
"Peter Childs" <peterachilds@gmail.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


2 Seams most likely  as they seam to occur more often when other when large queries (they are often followed by a record for a very very long query in a deferent transaction) or at particularly busy period when quite a lots of other short queries are also taking place.

I planning an upgrade to 8.3 once its out anyway so that might increase speed anyway.

Peter.

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