Most modern versions of UNIX report the percentage of time the CPU spends in each of four states. These are %System State, %User State, %Wait I/O and %Idle. The first two are reasonably straightforward - the CPU is actually busy and working in those states, either executing the code of user processes, or executing UNIX kernel code on behalf of user processes usually as a result of system calls. %Idle is also pretty easy to understand - the machine has 'nothing to do' and what you are seeing there is the percentage of time it spends in the Idle Process, a do-nothing loop which soaks up unused processor cycles. The fourth state is also an idle one, but it's one which has been forced on the machine. %Wait I/O is time in which the CPU could have been doing useful work, but all of its active processes are waiting for I/O operations to complete. This enforced idle time is degrading the ability of your system to do useful work, and should be avoided. Measured over reasonable periods (5-15 minutes), a %Wait I/O figure consistently above 10% is of concern. If you see this sort of thing, you will need to identify any disk drives which may be bottlenecks, i.e. those with high utilizations (> 40%). Metron's Athene is one of the tools that will enable you to do this. Moving filesystems around or re-arranging files to minimize head movement etc. will often produce beneficial results.