Quote Originally Posted by energyman View Post
not with raid6 where you can have 2 devices fail and still recover.
Sure, but you need a minimum of five disks before that's more efficient than mirroring.

And mirroring - what do you do when one disk reports the contents of block X as FOO but the other one as BAR. Which one has the correct block?
Generally speaking, if a disk fails then either the sector will be readable or it won't: with mirroring you can keep the readable sector and ignore the unreadable one. You could get the disks out of sync if they were writing during a power-down such that one disk was written to and the other wasn't, but that's a problem with any form of RAID which doesn't have battery backup.

Admittedly much of this could be worked around by smarter RAID implementations: i.e. don't fail the entire disk if one block is unreadable, but continue attempting to read the rest of the data as you rebuild the parity onto a new disk. Doesn't ZFS do something like that?