Re: shared_buffers advice

Greg Stark escribió:
> On Tue, Mar 16, 2010 at 2:30 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > "Pierre C" <lists@peufeu.com> writes:
> >> Does PG issue checkpoint writes in "sorted" order ?
> >
> > No.  IIRC, a patch for that was submitted, and rejected because no
> > significant performance improvement could be demonstrated.  We don't
> > have enough information about the actual on-disk layout to be very
> > intelligent about this, so it's better to just issue the writes and
> > let the OS sort them.
>
> Keep in mind that postgres is issuing writes to the OS buffer cache.
> It defers fsyncing the files as late as it can in the hopes that most
> of those buffers will be written out by the OS before then. That gives
> the OS a long time window in which to flush them out in whatever order
> and whatever schedule is most convenient.

Maybe it would make more sense to try to reorder the fsync calls
instead.

--
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.