On Thu, 2010-06-03 at 12:03 -0500, Bob Friesenhahn wrote:
> On Thu, 3 Jun 2010, David Dyer-Bennet wrote:
> >
> > In an 8-bay chassis, there are other concerns, too. Do I keep space open
> > for a hot spare? There's no real point in a hot spare if you have only
> > one vdev; that is, 8-drive RAIDZ3 is clearly better than 7-drive RAIDZ2
> > plus a hot spare. And putting everything into one vdev means that for any
> > upgrade I have to replace all 8 drives at once, a financial problem for a
> > home server.
>
> It is not so clear to me that an 8-drive raidz3 is clearly better than
> 7-drive raidz2 plus a hot spare. From a maintenance standpoint, I
> think that it is useful to have a spare drive or even an empty spare
> slot so that it is easy to replace a drive without needing to
> physically remove it from the system. A true hot spare allows
> replacement to start automatically right away if a failure is
> detected.
>
> With only 8-drives, the reliability improvement from raidz3 is
> unlikely to be borne out in practice. Other potential failures modes
> will completely drown out the on-paper reliability improvement
> provided by raidz3.
I tend to concur. I think that raidz3 is primarily useful in situations
with either an extremely large number of drives (very large arrays), or
in situations calling for extremely high fault tolerance (think
loss-of-life kinds of applications, or wall-street trading house
applications where downtime is measured in millions of dollars per
minute.)
And in those situations where raidz3 is called for, I think you still
want some pool of hot spares. (I'm thinking of the kinds of deployments
where the failure rate of drives approaches the ability of the site to
replace them quickly enough -- think very very large data centers with
hundreds or even thousands of drives.)
raidz3 is not, I think, for the typical home user, or even the typical
workgroup server application. I think I'd prefer raidz with hot
spare(s) over raidz2, even, for a typical situation. But I view raidz
in all its forms as a kind of compromise between redundancy,
performance, and capacity -- sort of a jack of all trades and master of
none. With $/Gb as low as they are today, I would be hard pressed to
recommend any of the raidz configurations except in applications calling
for huge amounts of data with no real performance requirements (nearline
backup kinds of applications) and no requirements for expandability.
(Situations where expansion is resolved by purchasing new arrays, rather
than growing storage within an array.)
-- Garrett
>
> Bob
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