Date: Wed, 9 Aug 2006 16:59:56 -0500
From: "Jim C. Nasby" <jnasby@pervasive.com>
To: Scott Marlowe <smarlowe@g2switchworks.com>
Cc: "Thomas F. O'Connell" <tfo@sitening.com>,
	"Joshua D.Drake" <jd@commandprompt.com>,
	Kenji Morishige <kenjim@juniper.net>, pgsql-performance@postgresql.org
Subject: Re: most bang for buck with ~ $20,000
Message-ID: <20060809215955.GB40481@pervasive.com>
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	<44D90FFE.7010807@commandprompt.com>
	<82C06632-148B-4489-90EB-BBB8A6BD1159@sitening.com>
	<1155136527.20252.90.camel@state.g2switchworks.com>
	<20060809213543.GV40481@pervasive.com>
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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