How to determine if your disk I/O sucks

If your I/O wait percentage is greater than (1/# of CPU cores) then your CPUs are waiting a significant amount of time for the disk subsystem to catch up.

Run the top command, if CPU I/O wait (wa) is say 13.9% and the server has 8 cores (1/8 cores = 0.125) then using the above statement, this is bad. Disk access may be slowing the application down if I/O wait is consistently around this threshold.