(cut)
> $ chdev -l hdisk1 -a q_type=simple -a queue_depth=3

topas -i 1


Disk Busy% KBPS TPS KB-Read KB-Writ
hdisk2 100.0 72192.0 282.0 0.0 72192.0


It works.
Thanks a lot.

with regards
Grzegorz