On 21:46, Lennert Buytenhek wrote:

While my RAID6 array was rebuilding after one disk had failed (which
I replaced), a second disk failed[*], and this caused the rebuild
process to start over from the beginning.

Why would the rebuild need to start over from the beginning in this
case?  Why couldn't it just continue from where it was?

I think this is because an easier algorithm may be used to rebuild from
a one disk failure than what is needed in case of a two disk failure.
In case of a one disk failure you can simply do the following:

rebuild P or Q: Just recompute it from the data disks
rebuild data: xor remaining data and P as in raid5 (in fact, the raid5
code is used in this case)

It's only the rebuild from a two disk failure that requires deeper
and more expensive math.

So if a second disk dies during the rebuild, the raid6 code must
switch from the easy algorithm to the "difficult" algorithm to finish
rebuilding the first failed disk. Theoretically this could be done
on the fly, but I think the current code doesn't do this now.

Andre
-- 
The only person who always got his work done by Friday was Robinson Crusoe