Re: Slow Inserts on large tables

Peter,

(please take this with a pinch of salt as I am no expert)

Here is  a possible scenario:
Each of your checkpoints takes 90 seconds or more (you told it  so with the checkpoint_completion_target).
If your insert fills 3 checkpoint segments (48 megs ) in less than 90 seconds then a new checkpoint request is issued. And maybe a third one, and so on. I imagine that this can flood the disk cache with write requests at some point although I can't explain how.
Have a look at the log, see the interval between the checkpoint requests and try to make this (a lot) larger than the checkpoint duration.
Start by increasing your checkpoint_segments (to, say, 16). If this doesn't work, maybe the timeout is too short, or the 90 seconds target to generous.

Regards,

Iulian

--- On Fri, 10/3/08, Peter Childs <peterachilds(at)gmail(dot)com> wrote:
From: Peter Childs <peterachilds(at)gmail(dot)com>
Subject: Re: [PERFORM] Slow Inserts on large tables
To:
Cc: "Postgresql Performance" <pgsql-performance(at)postgresql(dot)org>
Date: Friday, October 3, 2008, 9:47 AM

2008/10/3 Peter Eisentraut <peter_e(at)gmx(dot)net>:
> Peter Childs wrote:
>>
>> I have a problem where by an insert on a "large" table will
sometimes
>> take longer than usual.
>
>> I think the problem might have something to do with checkpoints,
>
> Then show us your checkpointing-related parameters. Or try to set them to
a
> lot higher values so checkpoints happen more rarely and see if that makes
a
> difference.
>
>

More often or less often?

I've currently got them set to

checkpoint_segments = 3
checkpoint_timeout = 180s
checkpoint_completion_target = 0.5

after reading that doing more smaller checkpoints might make each
checkpoint work quicker and hence less of a performance hit when they
actually happen.

Regards

Peter