On Jul 18, 2006, at 8:58, Richard Elling wrote:
Jeff Bonwick wrote:
For 6 disks, 3x2-way RAID-1+0 offers better resiliency than RAID-Z
or RAID-Z2.
Maybe I'm missing something, but it ought to be the other way around.
With 6 disks, RAID-Z2 can tolerate any two disk failures, whereas
for 3x2-way mirroring, of the (6 choose 2) = 6*5/2 = 15 possible
two-disk failure scenarios, three of them are fatal.
For the 6-disk case, with RAID-1+0 you get 27/64 surviving states
versus 22/64 for RAID-Z2. This accounts for the cases where you could
lose 3 disks and survive with RAID-1+0.
It seems to me that a useful resiliency calculation must include the
probability of the failures. Just because there are more potential
failure states for RAID-Z doesn't mean, in practical terms, at least,
that it is less resilient. Yes, there is one case of 3-disk failure
that the 3x2 arrangement will survive that RAID-Z2 won't, but there are
(as Jeff pointed out) three 2-disk failures that are fatal to 3x2.
Three different 2-failure scenarios total a much more likely occurrence
than than the net five (all requiring three or more failures) scenarios
that would be fatal to RAID-Z2 but not 3x2.
--Ed
_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
