Why is there only activity on one disk in RAID-5 configuration?

Question

I have a RHEL box running MySQL configured with 6x146GB disks in a RAID 5 configuration. However, when I do an "iostat -x 3" and view the activity on the devices, I only see activity on one of the disks while the others all show zeros for activity. This doesn't seem right to me. Here is some sample output:

avg-cpu:  %user   %nice    %sys %iowait   %idle
           0.25    0.00    0.29   15.71   83.75

Device:    rrqm/s wrqm/s   r/s   w/s  rsec/s  wsec/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await  svctm  %util
sda          0.00  28.67 247.67 311.00 2141.33 2717.33  1070.67  1358.67     8.70    11.73   21.00   1.44  80.37
dm-0         0.00   0.00  0.00  0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
dm-1         0.00   0.00  0.00  0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
dm-2         0.00   0.00  0.00  0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
dm-3         0.00   0.00  0.00  0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
dm-4         0.00   0.00  0.00  0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00   0.00   0.00
dm-5         0.00   0.00 247.67 339.67 2141.33 2717.33  1070.67  1358.67     8.27    11.81   20.11   1.37  80.33

Shouldn't there be activity across almost all the disks or at least two if there are writes occurring since the parity bit must be written out. Does iostat just not show actual physical disk activity when the disks are in RAID configuration?

Answer 1

You're mistaking the output of iostat, plus it looks like you are using hardware RAID which conceals the RAID activity because it presents only one physical device to the OS (/dev/sda)

I don't think dm-N refers to hard drives exactly, definitely not sub-items of a RAID array (I have dm-N, N=0..4 on a laptop that definitely has only one, non-RAIDed drive). My current belief is that they refer to I/O to or from devices with corresponding major numbers (available if you do an ls -l in /dev, as one of the first few columns) based on the man page for iostat.

If you want disk statistics for RAID components, you may need to ask the RAID hardware using whatever tool the manufacturer provides (if any), or switch to using software RAID.