Date: Thu, 9 Aug 2007 17:05:02 -0500
From: Decibel! <decibel@decibel.org>
To: Merlin Moncure <mmoncure@gmail.com>
Cc: Joe Uhl <joeuhl@gmail.com>, pgsql-performance@postgresql.org
Subject: Re: Dell Hardware Recommendations
Message-ID: <20070809220502.GE20424@nasby.net>
References: <46BB6F3D.6090608@gmail.com>
	<b42b73150708091450t448e3fa0nc778ab87d639603@mail.gmail.com>
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On Thu, Aug 09, 2007 at 05:50:10PM -0400, Merlin Moncure wrote:
> Raid 10 is usually better for databases but in my experience it's a
> roll of the dice.  If you factor cost into the matrix a SAS raid 05
> might outperform a SATA raid 10 because you are getting better storage
> utilization out of the drives (n - 2 vs. n / 2).  Then again, you
> might not.

It's going to depend heavily on the controller and the workload.
Theoretically, if most of your writes are to stripes that the controller
already has cached then you could actually out-perform RAID10. But
that's a really, really big IF, because if the strip isn't in cache you
have to read the entire thing in before you can do the write... and that
costs *a lot*.

Also, a good RAID controller can spread reads out across both drives in
each mirror on a RAID10. Though, there is an argument for not doing
that... it makes it much less likely that both drives in a mirror will
fail close enough to each other that you'd lose that chunk of data.

Speaking of failures, keep in mind that a normal RAID5 puts you only 2
drive failures away from data loss, while with RAID10 you can
potentially lose half the array without losing any data. If you do RAID5
with multiple parity copies that does change things; I'm not sure which
is better at that point (I suspect it matters how many drives are
involved).

The comment about the extra controller isn't a bad idea, although I
would hope that you'll have some kind of backup server available, which
makes an extra controller much less useful.
--=20
Decibel!, aka Jim Nasby                        decibel@decibel.org
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)

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