Can anyone give me a help in calculating the IOPS for my system please?

I had understood that IOPS could be calculated by monitoring IOSTAT. The r/s (reads/sec) and w/s (write/sec) combined basically gave you the peak IOPS.

However having monitored iostat for my app during a performance test, the figures I'm seeing lead me to believe that iostat couldn't be a reliable way to calculate IOPS. I get values of over 10,000 for w/s! I didn't think this was possible given the storage system i have (4 x 7500rpm sata disks in raid 5).

Can anyone help me? How *should* I calculate IOPS? How could I get over 10 thousand writes per second? Is it cahcing?

Thanks for any help,