Hello Neil, did you make some progress on this issue by any chance? I am hitting the same problem again on degraded RAID 6 missing two drives, kernel Debian 3.13.10-1, mdadm v3.2.5. Thanks. Patrik 2012-05-28 3:31 GMT+02:00 NeilBrown <neilb@xxxxxxx>: > > On Thu, 24 May 2012 14:37:28 +0200 Patrik Horník <patrik@xxxxxx> wrote: > > > On Thu, May 24, 2012 at 6:48 AM, NeilBrown <neilb@xxxxxxx> wrote: > > > > Firstly, degraded RAID6 with a left-symmetric layout is quite different from > > > an optimal RAID5 because there are Q blocks sprinkled around and some D > > > blocks missing. So there will always be more work to do. > > > > > > Degraded left-symmetric-6 is quite similar to optimal RAID5 as the same data > > > is stored in the same place - so reading should be exactly the same. > > > However writing is generally different and the code doesn't make any attempt > > > to notice and optimise cases that happen to be similar to RAID5. > > > > Actually I have left-symmetric-6 without one of the "regular" drives > > not the one with only Qs on it, so it should be similar to degraded > > RAID6 with a left-symmetric in this regard. > > Yes, it should - I had assumed wrongly ;-) > > > > > > A particular issue is that while RAID5 does read-modify-write when updating a > > > single block in an array with 5 or more devices (i.e. it reads the old data > > > block and the parity block, subtracts the old from parity and adds the new, > > > then writes both back), RAID6 does not. It always does a reconstruct-write, > > > so on a 6-device RAID6 it will read the other 4 data blocks, compute P and Q, > > > and write them out with the new data. > > > If it did read-modify-write it might be able to get away with reading just P, > > > Q, and the old data block - 3 reads instead of 4. However subtracting from > > > the Q block is more complicated that subtracting from the P block and has not > > > been implemented. > > > > OK, I did not know that. In my case I have 8 drives RAID6 degraded to > > 7 drives, so it would be plus to have it implemented the RAID5 way. > > But anyway I was thinking the whole-stripe detection should work in > > this case. > > > > > But that might not be the issue you are hitting - it simply shows that RAID6 > > > is different from RAID5 in important but non-obvious ways. > > > > > > Yes, RAID5 and RAID6 do try to detect whole-stripe write and write them out > > > without reading. This is not always possible though. > > > Maybe if you told us how many devices were in your arrays (which may be > > > import to understand exactly what is happening), what the chunk size is, and > > > exactly what command you use to write "lots of data". That might help > > > understand what is happening. > > > > The RAID5 is 5 drives, the RAID6 arrays are 7 of 8 drives, chunk size > > is 64K. I am using command dd if=/dev/zero of=file bs=X count=Y, it > > behaves the same for bs between 64K to 1 MB. Actually internal read > > speed from every drive is slightly higher that write speed, about cca > > 10%. The ratio between write speed to the array and write speed to > > individual drive is cca 5.5 - 5.7. > > I cannot really picture how the read speed can be higher than the write > speed. The spindle doesn't speed up for reads and slow down for writes does > it? But that's not really relevant. > > A 'dd' with large block size should be a good test. I just did a simple > experiment. With a 4-drive non-degraded RAID6 I get about a 1:100 ratio for > reads to writes for an extended write to the filesystem. > If I fail one device it becomes 1:1. Something certainly seems wrong there. > > RAID5 behaves more as you would expect - many more writes than reads. > > I've made a note to look into this when I get a chance. > > Thanks for the report. > > 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
