Re: RFC - Raid error detection and auto-recovery (was Fault tolerance with badblocks)

[Wols Lists <antlists@youngman.org.uk>]

On 15/05/17 12:11, Nix wrote:
> I think the point here is that we'd like some way to recover that lets
> us get back to the most-likely-consistent state. However, on going over
> the RAID-6 maths again I think I see where I was wrong. In the absence
> of P, Q, P *or* Q or one of P and Q and a data stripe, you can
> reconstruct the rest, but the only reason you can do that is because
> they are either correct or absent: you can trust them if they're there,
> and you cannot mistake a missing stripe for one that isn't missing.

The point of Peter Anvin's paper, though, was that it IS possible to
correct raid-6 if ONE of P, Q, or a data stripe is corrupt.

Elementary algebra. Given n unknowns, and n+1 independent facts about
them, we can solve for all unknowns.

With raid-5, we have P and the equation used to construct it, which
means we can solve for one *missing* block.

With raid-6, we have P, Q, and the equation, which means we can solve
for either *two* missing blocks, or *one* corrupt block and "which block
is corrupt?".

Cheers,
Wol