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 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
