On Tuesday December 16, david@xxxxxxxxxxxx wrote: > Hi Neil > > I brought this up in October but got no response - since you seem to be on a > roll I thought I'd try again... > > Summary: Add a spare and 'mirror-fail' a device. The spare is synced with the > to-be-removed device and any read errors are corrected from the remaining raid > devices. Once synced, the to-be-removed device is failed and the spare takes > its place. At no point is the array degraded. Yes, I've come to the conclusion that this probably is a good idea. See my 'road-map' that I just posted. Thanks, NeilBrown > > IMHO This one should be high on the todo list. Especially if it's a > pre-requisite for other improvements to resilience. > > Right now, if a drive fails or shows signs of going bad then you get into a very > risky situation. > > I'm sure most here know that the risk is because removing the failing drive and > installing a good one to re-sync puts you in a very vulnerable position; if > another drive fails (even one bad block) then you lose data. > > The solution involves raid1 - but it needs a twist of raid5/6 and it was > discussed ages ago; see: > http://arctic.org/~dean/proactive-raid5-disk-replacement.txt > > > I think this is what was discussed: > > Assume md0 has drives A B C D > D is failing > E is new > > * add E as spare > * set E to mirror 'failing' drive D (with bitmap?) > * subsequent writes go to both D+E > * recover 99+% of data from D to E by simple mirroring > * any read failures on D when reading from md0 or mirroring D->E are recovered > from reading ABC not E unless E is in sync. D is not failed out. (and it's these > tricks that stops users from doing all this manually) > * any md0 sector read failure on ABC can still (hopefully) be read from D even > if not yet mirrored to E (also not possible if done manually) > * once E is mirrored, D is removed and the job is done > > Personally I think this feature is more important than the reshaping requests; > of course that's just one opinion after replacing about 20 flaky 1Tb drives in > the past 6 months :) > > David > > -- > "Don't worry, you'll be fine; I saw it work in a cartoon once..." -- 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
