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.