Re: Large values for duration of COMMITs and slow queries. Due to large WAL config values?

On Sat, Nov 12, 2011 at 7:04 AM, Cody Caughlan <toolbag(at)gmail(dot)com> wrote:
> Postgres 9.1.1, master with 2 slaves via streaming replication.
>
> I've enabled slow query logging of 150ms and am seeing a large number
> of slow COMMITs:
>
> 2011-11-12 06:55:02 UTC pid:30897 (28/0-0) LOG:  duration: 232.398 ms
> statement: COMMIT
> 2011-11-12 06:55:08 UTC pid:30896 (27/0-0) LOG:  duration: 1078.789 ms
>  statement: COMMIT

Could well be related to checkpoints. Please try log_checkpoints = on
and see if there is a correlation.

> Recently we have bumped up wal_keep_segments and checkpoint_segments
> because we wanted to run long running queries on the slaves and we're
> receiving cancellation errors on the slaves. I think the master was
> recycling WAL logs from underneath the slave and thus canceling the
> queries. Hence, I believed I needed to crank up those values. It seems
> to work, I can run long queries (for statistics / reports) on the
> slaves just fine.

That reasoning isn't sound because it doesn't work like that.
Recycling WAL files has nothing to do with query cancelation on hot
standby.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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