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. 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? -- 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
