Re: long checkpoint_timeout

On Fri, Feb 23, 2007 at 10:14:29AM -0800, Jeff Davis wrote:
> The postgresql.conf says that the maximum checkpoint_timeout is 1 hour.
> However, the following messages seem to suggest that it may be useful to
> set the value significantly higher to reduce unnecessary WAL volume:
>
> http://archives.postgresql.org/pgsql-hackers/2006-10/msg00527.php
> http://archives.postgresql.org/pgsql-hackers/2006-08/msg01190.php
>
> Is there a reason for the hour-long limit on checkpoint_timeout? Is
> there a cost to doing so, aside from potentially longer recovery time?
>
> As I understand it, the background writer keeps the I/O more balanced
> anyway, avoiding I/O spikes at checkpoint.
>
> I don't need the checkpoint time to be higher than 1 hour, but I'm
> trying to understand the reasoning behind the limit and the implications
> of a longer checkpoint_timeout.
>
> The docs here:
>
> http://www.postgresql.org/docs/current/static/wal-configuration.html
>
> say that checkpoints cause extra disk I/O. Is there a good way to
> measure how much extra I/O (and WAL volume) is caused by the
> checkpoints? Also, it would be good to know how much total I/O is caused
> by a checkpoint so that I know if bgwriter is doing it's job.

There's a patch someone just came up with that provides additional debug
info about both bgwriter operation and checkpoints. I know it will at
least tell you how much was written out by a checkpoint.
--
Jim Nasby jim(at)nasby(dot)net
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)