Your Linux server is too slow or load is too high? One of the possible causes of such symptoms may be high IO (input/output) waiting time, which basically means that some of your processes need to read or write to a hard drive while it is too slow and not ready yet, serving data for some other processes.
Common practice is to use iostat -x in order to find out which block device (hard drive) is slow, but such information is not always much helpful. It could help you much more if you knew which process reads or writes the most data from your slow disk, so you could possibly renice it using ionice or even kill it.
IOTop will identify processes, which use high amount of input/output requests on your machine. It is similar to the well known top utility, but instead of showing you what consumes CPU the most, it lists processes by their IO consumption. Inspired by iotop python script from Guillaume Chazarain, but rewritten to C by Vyacheslav Trushkin so it now runs without python at all.