Re: shared_buffers documentation

Robert Haas wrote:
> On Mon, Apr 19, 2010 at 10:21 AM, Kevin Grittner
> <Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
> > Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> >
> >> 2. Reading the section on checkpoint_segments reminds me, again,
> >> that the current value seems extremely conservative on modern
> >> hardware. ?It's quite easy to hit this when doing large bulk data
> >> loads or even a big ol' CTAS. ?I think we should consider raising
> >> this for 9.1.
> >
> > Perhaps, but be aware the current default benchmarked better
> > than a larger setting in bulk loads.
> >
> > http://archives.postgresql.org/pgsql-hackers/2009-06/msg01382.php
> >
> > The apparent reason is that when there were fewer of them the WAL
> > files were re-used before the RAID controller flushed them from BBU
> > cache, causing an overall reduction in disk writes. ?I have little
> > doubt that *without* a good BBU cached controller a larger setting
> > is better, but it's not universally true that bigger is better on
> > this one.
>
> I don't actually know what's best. I'm just concerned that we have a
> default in postgresql.conf and a tuning guide that says "don't do
> that". Maybe the tuning guide needs to be more nuanced, or maybe
> postgresql.conf needs to be changed, but it makes no sense to have
> them saying contradictory things.

The good news about checkpoint_segments is that you get a log file
warning message if the value should be increased, i.e. you are
checkpointing often than 30 seconds.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com