> From: Peter Jeremy [mailto:peter.jer...@alcatel-lucent.com] > Sent: Thursday, October 07, 2010 10:02 PM > > On 2010-Oct-08 09:07:34 +0800, Edward Ned Harvey <sh...@nedharvey.com> > wrote: > >If you're going raidz3, with 7 disks, then you might as well just make > >mirrors instead, and eliminate the slow resilver. > > There is a difference in reliability: raidzN means _any_ N disks can > fail, whereas mirror means one disk in each mirror pair can fail. > With a mirror, Murphy's Law says that the second disk to fail will be > the pair of the first disk :-).
Maybe. But in reality, you're just guessing the probability of a single failure, the probability of multiple failures, and the probability of multiple failures within the critical time window and critical redundancy set. The probability of a 2nd failure within the critical time window is smaller whenever the critical time window is decreased, and the probability of that failure being within the critical redundancy set is smaller whenever your critical redundancy set is smaller. So if raidz2 takes twice as long to resilver than a mirror, and has a larger critical redundancy set, then you haven't gained any probable resiliency over a mirror. Although it's true with mirrors, it's possible for 2 disks to fail and result in loss of pool, I think the probability of that happening is smaller than the probability of a 3-disk failure in the raidz2. How much longer does a 7-disk raidz2 take to resilver as compared to a mirror? According to my calculations, it's in the vicinity of 10x longer. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
