On 09/05/17 13:27, Nix wrote: > On 9 May 2017, David Brown uttered the following: > (I'm not suggesting repairing RAID-5 mismatches. That's clearly > impossible. You can't even tell what disk is affected. But in the RAID-6 > case none of this is impossible, or so it seems to me. You have at least > three and probably four or more drives with consistent syndromes, and > one that is out of whack. You know which one must be wrong -- the > "minority vote" -- and you know what has to be done to make it > consistent with the others again. Why not do it? It's no more risky than > that aspect of a RAID rebuild from a failed disk would be.) > >> RAID will /not/ let you reliably detect or correct other sorts of >> errors. > > ... only it clearly can. What stops it from handling the RAID-6-and- > one-disk-is-wrong case where it cannot handle the RAID-6-and-one-disk- > has-failed case, given that you can unambiguously determine which disk > is wrong using the data on the surviving drives, with an undetected- > failure probability of something way below 2^128? (I could work out the > actual value but I haven't had any coffee yet and it seems pointless > when it's that low.) > >> What does /not/ work, however, is trying to squeeze magic capabilities >> out of existing layers in the system, or expecting more out of them that >> they can give. > > I don't see that these capabilities are any more magic than what RAID-6 > does already. It can recover from two failed drives: why can't it > recover from one wrong one? (Or, rather, from one drive with very > occasionally wrong sectors on it. Obviously if it was always getting > things wrong its presence is not a benefit and you have essentially > fallen back to nothing better than RAID-5, only with worse performance. > But that's what error thresholds are for, which md already employs in > similar situations.) > I thought you said that you had read Neil's article. Please go back and read it again. If you don't agree with what is written there, then there is little more I can say to convince you. One thing I can try, is to note that you are /not/ the first person to think "Surely with RAID-6 we can correct mismatches - it should be easy?". You are /not/ the first person to think "Correcting RAID-6 mismatches would be a marvellous feature that would make it /far/ better". Linux md raid does not correct RAID-6 mismatches found on a scrub. To my (admittedly limited) knowledge, hardware RAID-6 systems do not correct mismatches found on a scrub. If correcting RAID-6 mismatches were as simple, reliably, and useful as you seem to believe, than I think Linux md raid would already do it - either as part of the scrub, or as an extra utility to run on mismatched stripes. -- 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
