From: Gregory Stark <stark@enterprisedb.com>
To: pgsql-performance@postgresql.org
Subject: Re: setting up raid10 with more than 4 drives
In-Reply-To: <20070530142556.GB1785@mathom.us> (Michael Stone's message of
	"Wed, 30 May 2007 10:25:59 -0400")
Organization: EnterpriseDB
References: <a2de01dd0705300029t52ceb76fx25c3ad67db0c43c0@mail.gmail.com>
	<C282D30E.31B69%llonergan@greenplum.com>
	<20070530142556.GB1785@mathom.us>
Date: Wed, 30 May 2007 16:23:46 +0100
Message-ID: <87veea9vy5.fsf@oxford.xeocode.com>
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"Michael Stone" <mstone+postgres@mathom.us> writes:

"Michael Stone" <mstone+postgres@mathom.us> writes:

> On Wed, May 30, 2007 at 07:06:54AM -0700, Luke Lonergan wrote:
>
> > 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?  

Well, the one where the checksum is correct.

In practice I've never seen a RAID failure due to outright bad data. In my
experience when a drive goes bad it goes really bad and you can't read the
block at all without i/o errors.

In every case where I've seen bad data it was due to bad memory (in one case
bad memory in the RAID controller cache -- that was hell to track down).
Checksums aren't even enough in that case as you'll happily generate a
checksum for the bad data before storing it...

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