On 30/12/2011 21:04, Michele Codutti wrote:
[...]

This is one of many similar outputs from iostat -x 5 from the initial rebuilding phase:
avg-cpu:  %user   %nice %system %iowait  %steal   %idle
            0.00    0.00   13.29    0.00    0.00   86.71
Device: rrqm/s  wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util
sda    6585.60    0.00 4439.20    0.00 44099.20     0.00    19.87     6.14  1.38    1.38    0.00  0.09 39.28
sdb    6280.40    0.00 4746.60    0.00 44108.00     0.00    18.59     5.20  1.10    1.10    0.00  0.07 35.04
sdc       0.00 9895.40    0.00 1120.80     0.00 44152.80    78.79    12.03 10.73    0.00   10.73  0.82 92.32
I also build a RAID6 (with one drive missing): same results.

Hang on, are you saying you see the 40MB/s speeds during the initial 
rebuilding phase? Yes, you will get those results. You are seeing 
degraded mode performance in the RAID5 just as you are in the RAID6 with 
a missing drive. When the array is fully built, which may well take a 
day or two, you can expect better. Check /proc/mdstat for the progress 
of the initial build.

If you happen to know that your array is already in sync (which three 
brand new all-zero drives would be for RAID5), or want to test without 
waiting for a rebuild, you can use --assume-clean when creating the array.

Cheers,

John.