Date: Wed, 30 May 2007 10:25:59 -0400
From: Michael Stone <mstone+postgres@mathom.us>
Subject: Re: setting up raid10 with more than 4 drives
In-reply-to: <C282D30E.31B69%llonergan@greenplum.com>
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	<C282D30E.31B69%llonergan@greenplum.com>
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On Wed, May 30, 2007 at 07:06:54AM -0700, Luke Lonergan wrote:
>On 5/30/07 12:29 AM, "Peter Childs" <peterachilds@gmail.com> wrote:
>> Good point, also if you had Raid 1 with 3 drives with some bit errors at least
>> you can take a vote on whats right. Where as if you only have 2 and they
>> disagree how do you know which is right other than pick one and hope... But
>> whatever it will be slower to keep in sync on a heavy write system.
>
>Much better to get a RAID system that checksums blocks so that "good" is
>known.  Solaris ZFS does that, as do high end systems from EMC and HDS.

I don't see how that's better at all; in fact, it reduces to exactly the 
same problem: given two pieces of data which disagree, which is right?  
The ZFS hashes do a better job of error detection, but that's still not 
the same thing as a voting system (3 copies, 2 of 3 is correct answer) 
to resolve inconsistencies.

Mike Stone