Btrfs has been deprecated?

So how on earth does ZFS get around that issue? And anyway, no a mdraid does not necessarily cause that problem at all! Yes I would certainly advise against using raid-5 for a large (5 or more) disks array, because losing a disk puts the array in a state of no redundancy.

But firstly if the failed disk is even partially recoverable, that will massively reduce the stress on the array. (As always, we need developers - getting that to work seamlessly is on the hit list of jobs needing doing :-(

Secondly, if you've been scrubbing the array like you're supposed to, you know very well that the array is good and will have no trouble coping with a rebuild.

And thirdly, how on earth is ZFS supposed to know what is on the failed disk, other than by doing a scan of the rest of the disks, stressing them in exactly the same way as md-raid!?

(And yes, md-raid does scan the entire underlying partition like you suggest, but there are easy - if controversial - ways round that.)

Cheers,
Wol