From: Gregory Stark <stark@enterprisedb.com>
To: "Jonah H. Harris" <jonah.harris@gmail.com>
Cc: "Luke Lonergan" <llonergan@greenplum.com>,
	"Rajesh Kumar Mallah" <mallah.rajesh@gmail.com>,
	<pgsql-performance@postgresql.org>
Subject: Re: setting up raid10 with more than 4 drives
In-Reply-To: <36e682920705292141k9aa038btb543d97cee0fa4a1@mail.gmail.com>
	(Jonah H. Harris's message of "Wed, 30 May 2007 00:41:46 -0400")
Organization: EnterpriseDB
References: <a97c77030705291918j54b88259ia2f725e8b9e39dd6@mail.gmail.com>
	<C2823CFD.31AEB%llonergan@greenplum.com>
	<36e682920705292141k9aa038btb543d97cee0fa4a1@mail.gmail.com>
Date: Wed, 30 May 2007 13:08:25 +0100
Message-ID: <874plubjk6.fsf@oxford.xeocode.com>
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"Jonah H. Harris" <jonah.harris@gmail.com> writes:

> On 5/29/07, Luke Lonergan <llonergan@greenplum.com> wrote:
>> AFAIK you can't RAID1 more than two drives, so the above doesn't make sense
>> to me.

Sure you can. In fact it's a very common backup strategy. You build a
three-way mirror and then when it comes time to back it up you break it into a
two-way mirror and back up the orphaned array at your leisure. When it's done
you re-add it and rebuild the degraded array. Good raid controllers can
rebuild the array at low priority squeezing in the reads in idle cycles.

I don't think you normally do it for performance though since there's more to
be gained by using larger stripes. In theory you should get the same boost on
reads as widening your stripes but of course you get no benefit on writes. And
I'm not sure raid controllers optimize raid1 accesses well in practice either.

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com