Subject: long checkpoint_timeout
From: Jeff Davis <pgsql@j-davis.com>
To: pgsql-performance@postgresql.org
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Date: Fri, 23 Feb 2007 10:14:29 -0800
Message-Id: <1172254469.10824.243.camel@dogma.v10.wvs>
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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.

Regards,
	Jeff Davis