Re: XLOG_BLCKSZ vs. wal_buffers table

On Tue, 02 May 2006 10:52:38 +0100
Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:

> On Sun, 2006-04-30 at 22:14 -0700, Mark Wong wrote:
> > I would have gotten this out sooner but I'm having trouble with our
> > infrastructure. Here's a link to a table of data I've started putting
> > together regarding XLOG_BLCKSZ and wal_buffers on a 4-way Opteron
> > system:
> > http://developer.osdl.org/markw/pgsql/xlog_blcksz.html
> >
> > There are a couple of holes in the table but I think it shows enough
> > evidence to say that with dbt2 having a larger XLOG_BLCKSZ improves the
> > overall throughput of the test.
> >
> > I'm planning on continuing to increase XLOG_BLCKSZ and wal_buffers to
> > determine when the throughput starts to level out or drop off, and then
> > start experimenting with varying BLCKSZ. Let me know if there are other
> > things that would be more interesting to experiment with first.
>
> IMHO you should be testing with higher wal_buffers settings. ISTM likely
> that the improved performance is due to there being more buffer space,
> rather than actually improving I/O. Setting wal_buffers to something
> fairly high say 4096 would completely remove any such effect so we are
> left with a view on the I/O.

I ran another few tests at the 600 scale factor just in case I was
getting close to peaking at 500 warehouses. (Link above has updated
data.) With wal_buffers set to 4096 the difference between 2048, 8192,
and 32768 seem negligible. Some of the disks are at 90% utilization so
perhaps I need to take a close look to make sure none of the other
system resources are pegged.

Thanks,
Mark