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...0/msg00527.php
http://archives.postgresql.org/pgsql...8/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/curre...iguration.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


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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...0/msg00527.php
> http://archives.postgresql.org/pgsql...8/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/curre...iguration.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@nasby.net
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)

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On Fri, 2007-02-23 at 14:02 -0600, Jim C. Nasby wrote:
> > 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.

Excellent, that would answer a lot of my questions. I did some brief
searching and nothing turned up. Do you have a link to the discussion or
the patch?

Regards,
Jeff Davis


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On Fri, Feb 23, 2007 at 12:23:08PM -0800, Jeff Davis wrote:
> On Fri, 2007-02-23 at 14:02 -0600, Jim C. Nasby wrote:
> > > 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.
>
> Excellent, that would answer a lot of my questions. I did some brief
> searching and nothing turned up. Do you have a link to the discussion or
> the patch?

http://archives.postgresql.org/pgsql...2/msg01083.php
--
Jim Nasby jim@nasby.net
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)

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