If you look at /proc/diskstats and particularly the 4th and 6th fields for
the device that you are interested in, and then take the differences for each
field between 'before' and 'after' running a test you will get
- the number of IO requests
 - the number of sectors

that were serviced during that time.  Taking a ratio will get you the number
of sectors per IO.  Normally more is better.

Hi Neil,
are you sure the right value are the 4th and the 6th fields ? I see strange 
value in them (the 4th field is bigger then the 6th while i was expecting the 
contrary)
looking in the iostats.txt file (kernel documentation) i suspect the fields you 
are interested are the first and the third.
In this hypothesis (1st and 3rd fields) the ratio i obtain are:

sda 84.02 
sdb 113,69
md2 8,01

md2 and his meber have a very different ratio....

It might be interesting to find out what the data offset is for your RAID1
(mdadm --examine will tell you if there is one), and compare the
request/sector numbers and see if they show anything.

this is the output of mdadm --examine /dev/sda3

/dev/sda3:
          Magic : a92b4efc
        Version : 0.90.00
           UUID : d7ca6fdd:8cf9e3ed:bea46eb0:98c63a97
  Creation Time : Tue Sep 23 15:49:29 2008
     Raid Level : raid1
  Used Dev Size : 237826176 (226.81 GiB 243.53 GB)
     Array Size : 237826176 (226.81 GiB 243.53 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 2

    Update Time : Tue Jul 27 18:07:41 2010
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0
       Checksum : 8afc34a - correct
         Events : 18156


      Number   Major   Minor   RaidDevice State
this     0       8        3        0      active sync   /dev/sda3

   0     0       8        3        0      active sync   /dev/sda3
   1     1       8       19        1      active sync   /dev/sdb3

Do you have some hypothesis ?

thank you all

Marco