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Re: SAN performance mystery

From: "Alex Turner" <armtuk(at)gmail(dot)com>
To: "Mark Lewis" <mark(dot)lewis(at)mir3(dot)com>
Cc: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "Brian Hurt" <bhurt(at)janestcapital(dot)com>, "Tim Allen" <tim(at)proximity(dot)com(dot)au>, pgsql-performance(at)postgresql(dot)org
Subject: Re: SAN performance mystery
Date: 2006-06-15 23:58:00
Message-ID: 33c6269f0606151658h731915d3k405b4da67c49228a@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
Given the fact that most SATA drives have only an 8MB cache, and your RAID
controller should have at least 64MB, I would argue that the system with the
RAID controller should always be faster.  If it's not, you're getting
short-changed somewhere, which is typical on linux, because the drivers just
aren't there for a great many controllers that are out there.

Alex.

On 6/15/06, Mark Lewis <mark(dot)lewis(at)mir3(dot)com> wrote:
>
> On Thu, 2006-06-15 at 18:24 -0400, Tom Lane wrote:
> > I agree with Brian's suspicion that the SATA drive isn't properly
> > fsync'ing to disk, resulting in bogusly high throughput.  However,
> > ISTM a well-configured SAN ought to be able to match even the bogus
> > throughput, because it should be able to rely on battery-backed
> > cache to hold written blocks across a power failure, and hence should
> > be able to report write-complete as soon as it's got the page in cache
> > rather than having to wait till it's really down on magnetic platter.
> > Which is what the SATA drive is doing ... only it can't keep the promise
> > it's making for lack of any battery backup on its on-board cache.
>
> It really depends on your SAN RAID controller.  We have an HP SAN; I
> don't remember the model number exactly, but we ran some tests and with
> the battery-backed write cache enabled, we got some improvement in write
> performance but it wasn't NEARLY as fast as an SATA drive which lied
> about write completion.
>
> The write-and-fsync latency was only about 2-3 times better than with no
> write cache at all.  So I wouldn't assume that just because you've got a
> write cache on your SAN, that you're getting the same speed as
> fsync=off, at least for some cheap controllers.
>
> -- Mark Lewis
>
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