On Thu, June 3, 2010 12:03, 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.
But is having a RAIDZ2 drop to single redundancy, with replacement starting instantly, actually as good or better than having a RAIDZ3 drop to double redundancy, with actual replacement happening later? The "degraded" state of the RAIDZ3 has the same redundancy as the "healthy" state of the RAIDZ2. Certainly having a spare drive bay to play with is often helpful; though the scenarios that most immediately spring to mind are all mirror-related and hence don't apply here. > 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 wouldn't give up much of anything to add Z3 on 8 drives, no. -- David Dyer-Bennet, d...@dd-b.net; http://dd-b.net/ Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/ Photos: http://dd-b.net/photography/gallery/ Dragaera: http://dragaera.info _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
