Re: Using pgiosim realistically

On Sat, May 14, 2011 at 12:07:02PM -0500, ktm(at)rice(dot)edu wrote:
> On Fri, May 13, 2011 at 09:09:41PM +0000, John Rouillard wrote:
> > I am adding pgiosim to our testing for new database hardware and I am
> > seeing something I don't quite get and I think it's because I am using
> > pgiosim incorrectly.
> >
> > Specs:
> >
> > OS: centos 5.5 kernel: 2.6.18-194.32.1.el5
> > memory: 96GB
> > cpu: 2x Intel(R) Xeon(R) X5690 @ 3.47GHz (6 core, ht enabled)
> > disks: WD2003FYYS RE4
> > raid: lsi - 9260-4i with 8 disks in raid 10 configuration
> > 1MB stripe size
> > raid cache enabled w/ bbu
> > disk caches disabled
> > filesystem: ext3 created with -E stride=256
> >
> > I am seeing really poor (70) iops with pgiosim. According to:
> > http://www.tomshardware.com/reviews/2tb-hdd-7200,2430-8.html in the
> > database benchmark they are seeing ~170 iops on a single disk for
> > these drives. I would expect an 8 disk raid 10 should get better then
> > 3x the single disk rate (assuming the data is randomly distributed).
> Those drives are 7200 rpm drives which would give you a maximum write
> rate of 120/sec at best with the cache disabled. I actually think your
> 70/sec is closer to reality and what you should anticipate in real use.
> I do not see how they could make 170/sec. Did they strap a jet engine to
> the drive. :)

Hmm, I stated the disk cache was disabled. I should have said the disk
write cache, but it's possible the readhead cache is disabled as well
(not quite sure how to tell on the lsi cards). Also there isn't a lot
of detail in what the database test mix is and I haven't tried
researching the site to see if the spec the exact test. If it included
a lot of writes and they were being handled by a cache then that could
explain it.

However, in my case I have an 8 disk raid 10 with a read only load (in
this testing configuration). Shouldn't I expect more iops than a
single disk can provide? Maybe pgiosim is hitting some other boundary
than just i/o?

Also it turns out that pgiosim can only handle 64 files. I haven't
checked to see if this is a compile time changable item or not.

--
-- rouilj

John Rouillard System Administrator
Renesys Corporation 603-244-9084 (cell) 603-643-9300 x 111