I just replaced a fault drive in my raid6-array (consisting of 8 drives) during the recovery I did notice something strange in iostat. All drives are getting the same speeds (as expected) except for one drive (sdi) which is constantly reading faster than the rest.
It is also reading about one eighth faster which might have something to do with that there are in total eight drives in the array, but I don't know why...
This was true during the whole recovery (always the same drive reading faster than all of the rest) and looking at the total statistics for the recovery all drives have read/written pretty much the same amount except for sdi which have read one eighth more.
Some iostat stats averaged for 100s:
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
sdb 444.80 26.15 0.00 2615 0
sdb1 444.80 26.15 0.00 2615 0
sdc 445.07 26.15 0.00 2615 0
sdc1 445.07 26.15 0.00 2615 0
sdd 443.21 26.15 0.00 2615 0
sdd1 443.21 26.15 0.00 2615 0
sde 444.01 26.15 0.00 2615 0
sde1 444.01 26.15 0.00 2615 0
sdf 448.79 26.15 0.00 2615 0
sdf1 448.79 26.15 0.00 2615 0
sdg 521.66 0.00 26.15 0 2615
sdg1 521.66 0.00 26.15 0 2615
sdh 443.32 26.15 0.00 2615 0
sdh1 443.32 26.15 0.00 2615 0
sdi 369.23 29.43 0.00 2942 0
sdi1 369.23 29.43 0.00 2942 0
Can anyone offer a sensible explanation? When I discovered that it was ~exactly one eighth faster I figured that it had to do with the parity but that really didn't make much sense (I don't know about the specific raid 6 implementation in mdadm but for one it surely can't store all of the parity on one drive...).
UPDATE: Well, I did replace another drive just now (same array) and I am seeing the exact same results but this time with a different drive reading faster (actually, it is the drive I added for the last recovery that has decided it want to do more work).
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
sdb 388.48 24.91 0.00 2490 0
sdb1 388.48 24.91 0.00 2490 0
sdc 388.13 24.91 0.00 2491 0
sdc1 388.13 24.91 0.00 2491 0
sdd 388.32 24.91 0.00 2491 0
sdd1 388.32 24.91 0.00 2491 0
sde 388.81 24.91 0.00 2491 0
sde1 388.81 24.91 0.00 2491 0
sdf 501.07 0.00 24.89 0 2489
sdf1 501.07 0.00 24.89 0 2489
sdg 356.86 28.03 0.00 2802 0
sdg1 356.86 28.03 0.00 2802 0
sdh 387.52 24.91 0.00 2491 0
sdh1 387.52 24.91 0.00 2491 0
sdi 388.79 24.92 0.00 2491 0
sdi1 388.79 24.92 0.00 2491 0
Theese are 4k-drives (but as all drives do (or at least did) they stil report 512-byte sectors). So I figured I might have aligned the partitions wrong somehow (what implications that might have had I don't know, depends on how mdadm works and stripe size I guess, anyway easy enough to check):
debbie:~# fdisk -l -u /dev/sd[bcdefghi] | grep ^/dev/sd
/dev/sdb1 2048 3906988207 1953493080 fd Linux raid autodetect
/dev/sdc1 2048 3906988207 1953493080 fd Linux raid autodetect
/dev/sdd1 2048 3906988207 1953493080 fd Linux raid autodetect
/dev/sde1 2048 3906988207 1953493080 fd Linux raid autodetect
/dev/sdf1 2048 3907024064 1953511008+ fd Linux raid autodetect
/dev/sdg1 2048 3907024064 1953511008+ fd Linux raid autodetect
/dev/sdh1 2048 3906988207 1953493080 fd Linux raid autodetect
/dev/sdi1 2048 3906988207 1953493080 fd Linux raid autodetect
f and g are the new drives and appear to be slightly bigger but they all start on the same sector (all drives are of the same make an model (and on the same controller) but the new ones are bought ~6 months later than the rest).
