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@xxxxxxxxx EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)
