On Tue, Jan 30 2018, David Brown wrote: > Does anyone know the current state of multi-layer raid (in the Linux md > layer) for recovery? > > I am thinking of a setup like this (hypothetical example - it is not a > real setup): > > md0 = sda + sdb, raid1 > md1 = sdc + sdd, raid1 > md2 = sde + sdf, raid1 > md3 = sdg + sdh, raid1 > > md4 = md0 + md1 + md2 + md3, raid5 > > > If you have an error reading a sector in sda, the raid1 pair finds the > mirror copy on sdb, re-writes the data to sda (which re-locates the bad > sector) and passes the good data on to the raid5 layer. Everyone is > happy, and the error is corrected quickly. > > Rebuilds are fast as single disk copies. > > > However, if you have an error reading a sector in sda /and/ when reading > the mirror copy in sdb, then the raid1 pair has no data to give to the > raid5 layer. The raid5 layer will then read the rest of the stripe and > calculate the missing data. I presume it will then re-write the > calculated data to md0, which will in turn write it to sda and sdb, and > all will be well again. If sda and sdb have bad-block-logs configured, this should work. Not everyone trusts them though. > > > But what about rebuilds? A rebuild or recovery of the raid1 layer is > not triggered by a read from the raid5 level - it will be handled at the > raid1 level. If sda is replaced, then the raid1 level will build it by > copying from sdb. If a read error is encountered while copying, is > there any way for the recovery code to know that it can get the missing > data by asking the raid5 level? Is it possible to mark the matching sda > sector as bad, so that a future raid5 read (such as from a scrub) will > see that md0 stripe as bad, and re-write it? > "Is it possible to mark the matching sda sector as bad" This is exactly what the bad-block-list functionality is meant to do. NeilBrown > > > > -- > 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
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