On Wed, Aug 09, 2006 at 04:50:30PM -0500, Scott Marlowe wrote:
> On Wed, 2006-08-09 at 16:35, Jim C. Nasby wrote:
> > On Wed, Aug 09, 2006 at 10:15:27AM -0500, Scott Marlowe wrote:
> > > Actually, the BIGGEST win comes when you've got battery backed cache on
> > > your RAID controller. In fact, I'd spend money on a separate RAID
> > > controller for xlog with its own cache hitting a simple mirror set
> > > before I'd spring for more drives on pg_xlog. The battery backed cache
> > > on the pg_xlog likely wouldn't need to be big, just there and set to
> > > write-back.
> > >
> > > Then put all the rest of your cash into disks on a big RAID 10 config,
> > > and as big of a battery backed cache as you can afford for it and memory
> > > for the machine.
> >
> > Actually, my (limited) testing has show than on a good battery-backed
> > controller, there's no penalty to leaving pg_xlog in with the rest of
> > PGDATA. This means that the OP could pile all 8 drives into a RAID10,
> > which would almost certainly do better than 6+2.
>
> I've seen a few posts that said that before. I wonder if there's a
> point where the single RAID array / controller would get saturated and a
> second one would help. I think most of the testing I've seen so far has
> been multiple RAID arrays under the same controller, hasn't it?
Yeah. I've had one client try it so far, but it was a pretty small array
(8 drives, IIRC).
I suspect that by the time you get to a size where you're saturating a
controller, you're looking at enough drives where having two extra
(instead of dedicating them to pg_xlog) won't make much difference.
> > Note that some controllers (such as 3ware) need to periodically test the
> > life of the BBU, and they disable write caching when they do so, which
> > would tank performance.
>
> ugh, that's a scary thing. Can you at least schedule it?
Yeah, it's not automatic at all. Which itself is somewhat scarry....
--
Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461
