disk usage statistics

[Michal]

Hello.

Could you suggest me how can I get my disk usage statistics in terms of
percentage of the possible disk activity (like in gstat) or megabytes
per second (like in iostat), please?

I need something that gives actual usage statistics, not any averages.
And something that prints what I want and quits (not like gstat in
default mode). It have to be available for unprivileged user and I don't
want to use any temporally files in the process.

The closest I can get is (last column of):
iostat -c 2 -d ad0 | tail -n 1

Problem is that it takes two seconds, which is not acceptable for me
because I want it's output to be printed in status bar beside date and 
time. I'm using wmii window manager.

Any suggestions and hints are very welcome.
Michal
-- 
"Never interrupt your enemy when he is making a mistake." -Napoleon 
Bonaparte