Tomasz Chmielewski wrote:

If you start using your drives for two "dd if=bigfile[12] 
of=/dev/null" at the same time, you will notice the difference.

OT: Does this dd line really work? I did see it elsewhere already but 
doesn't appear to work for me: seems to see only the last if=. I don't 
think multiple input or output files are supported.
For this kind of tests I was doing a for loop with background dd comands 
( trailing & ) IIRC
This would be a good dd feature to simplify linux-raid performance 
testing though.