On Mon, Jan 28, 2013 at 03:47:13PM -0700, Chris Murphy wrote: [...] > In RAID 6 are the two parity chunks in a stripe identical? If yes, does 'unknown' only tell us that the data chunk recomputed parity does not match either of the parity chunks; or does it tell us anything about whether the parity chunks themselves are still the same or different? Hi Chris, the two parities, P and Q, are not the same, they're different Reed-Solomon polynomials. You'll find more detail here: http://www.kernel.org/pub/linux/kernel/people/hpa/raid6.pdf At the end of this document, it is explained what I've implemented in raid6check. The process is the following. The data is read (per stripe) and P, Q are then calculated. If the read P and Q are the same as the calculated ones, then it is *assumed* the data is OK. If P *or* (xor) Q does not match, then it is *assumed* the non matching one is wrong. If both, P and Q, do not match, then there is a calculation which returns us the slot (chunk position, if you want) where the error could be. If the slot is not in the ammissible range, for example if we have 5 data disk (0 to 4) and the calculation returns 10, then it means the error cannot be detected, which, in turn, means probably more than one slot has errors. The "unknown" above states exactly this, the two parities are not matching (P wrong, Q wrong at the same position) and the calculation cannot find which slot could be the corrupted one, so there could be more than one and, with only two parities, we cannot say anything more. > I think regression is going to be needed to find it. Hopefully the problem is restricted to parity computation and data chunks aren't affected; however if a URE occurs in a data chunk, it could be reconstructed incorrectly from bad parity so it's obviously still a big problem. Maybe parities are not correctly calculated, but I wonder how this could be, given the amount of usage the code had and has. Unless the new VX optimization have a problem. bye, -- piergiorgio -- 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
