On Wed, 2006-06-28 at 17:32, Erik Trimble wrote: > The main reason I don't see ZFS mirror / HW RAID5 as useful is this: > > ZFS mirror/ RAID5: capacity = (N / 2) -1 > speed << N / 2 -1 > minimum # disks to lose before loss > of data: 4 > maximum # disks to lose before loss > of data: (N / 2) + 2 > > ZFS mirror / HW Stripe capacity = (N / 2) > speed >= N / 2 > minimum # disks to lose before loss > of data: 2 > maximum # disks to lose before loss > of data: (N / 2) + 1 > > Given a reasonable number of hot-spares, I simply can't see the (very) > marginal increase in safety give by using HW RAID5 as out balancing the > considerable speed hit using RAID5 takes.
That's not quite right. There's no significant difference in performance, and the question is whether you're prepared to give up a small amount of space for orders of magnitude increase in safety. Each extra disk you can survive the failure of leads to a massive increase in safety. By something like (just considering isolated random disk failures) the ratio of the MTBF of a disk to the time it takes to get a spare back in and repair the LUN. That's something like 100,000 - which isn't a marginal increase in safety! Yes, I know it's not that simple. The point to take from this is simply that being able to survive 2 failures instead of 1 doesn't double your safety, it increases it by a very large number. And by a very large number again for the next failure you can survive. In the stripe case, a single disk loss (pretty common) loses all your redundancy straight off. A second disk failure (not particularly rare) and all your data's toast. Hot spares don't really help in this case. At this point having HW raid-5 underneath means you're still humming along safely. -- -Peter Tribble L.I.S., University of Hertfordshire - http://www.herts.ac.uk/ http://www.petertribble.co.uk/ - http://ptribble.blogspot.com/ _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
