David Magda wrote:
On Jan 30, 2007, at 09:52, Luke Scharf wrote:"Hey, I can take a double-drive failure now! And I don't even need to rebuild! Just like having a hot spare with raid5, but without the rebuild time!"Theoretically you want to rebuild as soon as possible, because running in degraded mode (even with dual-parity) increases your chances of data loss (even though the probabilities involved may seem remote). Case in point, recently at work we had a drive fail in a server with 5+1 RAID5 configuration. We replaced it, and about 2-3 weeks later a separate drive failed. Even with dual-parity, if we hadn't replaced / rebuilt things we would now be cutting it close.
I did misspeak -- with raidz2, I still do have to replace the failed drive ASAP!
However, with raidz2, you don't have to wait hours for the rebuild to occur before the second drive can fail; with a hot-spare, the first and second failures (provided that the failures occur on the array-drives, rather than on the spare) must happen several hours apart. With raidz2 on the same hardware, the two failures can happen at the same time -- and the array can still be rebuilt.
But, I guess the utility of the hot-spare depends a lot on the number of drives available, and on the layout. In my case, most of the hardware that I have is Apple XRaid units and, when using the hardware RAID inside the unit, the hot-spare must be in the same half of the box as failed drive -- in these small, constrained RAIDs, raidz2 would be much better than raidz and a spare because of the rebuild-time. With Thumper+ZFS or something like that, though, the spare could be anywhere, and I think I'd like having a few hot/warm spares on the machine that could be zinged into service if an array member fails.
-Luke
smime.p7s
Description: S/MIME Cryptographic Signature
_______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
