iostat numbers wrong when used with mdadm?

Question

See the nubmers below for /dev/sdb1, /dev/sdc1, /dev/sdd1, /dev/sde1:

$ iostat -dxm && cat /proc/mdstat
Linux 3.5.0-17-generic (avarice)    12-12-29    _x86_64_    (2 CPU)

Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda               4.29     3.97    5.48    6.55     0.25     0.08    55.17     0.42   34.63   63.87   10.18   4.55   5.47
sdb            2717.50     0.00   54.54    0.01    10.83     0.00   406.53     0.14    2.53    2.53    6.53   1.63   8.92
sdc            1390.51     0.00   11.67    0.01     5.48     0.00   960.60     0.04    3.00    3.00    5.47   1.57   1.83
sdd            1390.49     0.00   11.50    0.01     5.48     0.00   974.54     0.04    3.06    3.06    6.13   1.61   1.85
sde               0.10  1390.35    0.44   11.03     0.00     5.47   977.75     0.03    2.68    0.31    2.77   2.57   2.95
md0               0.00     0.00    0.05    0.00     0.00     0.00     8.00     0.00    0.00    0.00    0.00   0.00   0.00

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
md0 : active raid5 sde1[4] sdd1[2] sdc1[1] sdb1[0]
      5860145664 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [UUU_]
      [>....................]  recovery =  0.5% (10476048/1953381888) finish=501.1min speed=64609K/sec

The RAID rebuild is running at 64MB/s. iostat says that the associated drives are reading/writing at about 5MB/s. Is iostat wrong?

Answer 1

To me those numbers look like they are in the vague ballpark for being right for a RAID-5 rebuild.

Yes it is only writing 5Mb/s to sde, but in order to write that data it needs to read a substantial amount more data from the other drives first to compare and restore the missing drive data. Due to your array being RAID-5 I would expect the ratio of read to write to be of the order of 3/4 reading and 1/4 writing.

Granted 5MB/s seems a little low, but I do not know if the output from lost at is instantaneous and you just caught it at a bad moment.