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Re: SAN performance mystery

From: Mark Lewis <mark(dot)lewis(at)mir3(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: 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 23:25:17
Message-ID: 1150413917.31200.109.camel@archimedes (view raw or flat)
Thread:
Lists: pgsql-performance
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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