There is a nifty little program called 'iotop'. Iotop is part of Debian or Ubuntu and can be installed with a simple apt-get.
Once you have determined with 'top' that the system is waiting on IO-access, It is nice to know
which process is responsibe for this IO. Therefore, you want a list of processes, just like top provides, but instead of CPU or RAM usage, it shows the IO a process is generating. This is exactly what 'iotop' provides.
This is ideal when troubleshooting performance problems caused by heavy IO.