So much nicer with ionice

Use ionice to give different processes more priority on reading and writing to your file system. A good example is the Transmission torrent client, which I prefer to run in the background most of the time. You have to get the Process ID (PID) by using the System Monitor, which can be ran from the Administration menu or by pressing ALT+F2 gnome-system-monitor. Go to the process list and find the name of the program (transmission in this example), copy the PID, and then press ALT+F2 gksudo ionice -c 3 -p 1234 where 1234 is instead the PID. I used 3 here to make it use Idle scheduling, but you can use 1, 2, and 3 for varying degrees. See man ionice for how that works in detail.

from: http://santiance.com/tag/ubuntu/