Re: [zfs-discuss] Big JBOD: what would you do?

[Gregory Shaw]

On Jul 18, 2006, at 10:35 AM, Ed Gould wrote: 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 To add to Ed's comments: It would be good to add serviceability to the picture as well.  If we can detect a failed disk and fix it without impact, the probability of a multi-disk failure decreases. When it comes to cascade failures (lots of disks going bad, whether that be real (bad disks) or perceived (i/o subsystem failures)), the only real solution is to use discrete disk solutions with separate power, controllers, etc.   It goes back to the ILM model.   If the value of your data justifies multiple disk subsystems, so be it.  If it doesn't, and you have a cascade failure, I hope your backups are intact. ----- Gregory Shaw, IT Architect Phone: (303) 673-8273        Fax: (303) 673-2773 ITCTO Group, Sun Microsystems Inc. 1 StorageTek Drive ULVL4-382              [EMAIL PROTECTED] (work) Louisville, CO 80028-4382                    [EMAIL PROTECTED] (home) "When Microsoft writes an application for Linux, I've Won." - Linus Torvalds

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