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Re: setting up raid10 with more than 4 drives

From: Gregory Stark <stark(at)enterprisedb(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: setting up raid10 with more than 4 drives
Date: 2007-05-30 15:23:46
Message-ID: 87veea9vy5.fsf@oxford.xeocode.com (view raw or flat)
Thread:
Lists: pgsql-performance
"Michael Stone" <mstone+postgres(at)mathom(dot)us> writes:

"Michael Stone" <mstone+postgres(at)mathom(dot)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


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