On 07/11/19 13:17, Xiao Ni wrote: > > > On 11/05/2019 08:33 AM, Wols Lists wrote: >> On 04/11/19 20:01, Nigel Croxon wrote: >>> The MD driver for level-456 should prevent re-reading read errors. >>> >>> For redundant raid it makes no sense to retry the operation: >>> When one of the disks in the array hits a read error, that will >>> cause a stall for the reading process: >>> - either the read succeeds (e.g. after 4 seconds the HDD error >>> strategy could read the sector) >>> - or it fails after HDD imposed timeout (w/TLER, e.g. after 7 >>> seconds (might be even longer) >> Okay, I'm being completely naive here, but what is going on? Are you >> saying that if we hit a read error, we just carry on, ignore it, and >> calculate the missing block from parity? >> >> If so, what happens if we hit two errors on a raid-5, or 3 on a raid-6, >> or whatever ... :-) >> > Hi Wol > > What's the meaning of "two errors on a raid-5"? Two read errors happen > on one disk? > Or there are two read errors on two disks? > Two read errors on two disks, so that you can't recalculate from parity. Basically, what I was thinking was "does this patch mean that if we get a read error, we read the parity instead and recalculate the block that failed?". If that is the case, what happens if we get a second read error and can't recalculate? Because, aiu real-world behaviour, it's quite normal for the first read to fail and the retry to succeed. So if this patch does what I think (feel free to tell me I'm wrong :-) a double read error would make raid return a read error, when actually the old code would have resulted in the read being successful, if a bit slower. Cheers, Wol
