"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