performance of raid10,f2 on 4 disks

["Keld Jørn Simonsen" <keld@dkuug.dk>]

Hi!

I have played around with raid10,f2 on a 2 disk array set,
and I really liked the performance on the sequential reads.
It looked like double up on the speed, about 173 MB/s
for two SATA-2 disks. 

I then went on to look at my 4 new SATS-2 disks, to have
the same kind of performance I made the array by:

mdadm --create /dev/md3 --chunk=256 -R -l 10 -n 4 -p f2 /dev/sd[abcd]1

And my first tests showed a sequential read rate of 320 MB/s.
Impressive! I then tried it a few more times, but then I could not
get more than around 160 MB/s, which is less than what I got on 2 disks.

Any ideas of what is going on?

Best regards
keld