ionice program sets or gets the io scheduling class and priority for a program. If no arguments or just -p is given, ionice will query the current io scheduling class and priority for that process.
A process can be in one of three scheduling classes:
Idle
Continue Reading... A process can be in one of three scheduling classes:
Idle
A program running with idle io priority will only get disk time when no other program has asked for disk io for a defined grace period. The impact of idle io processes on normal system activity should be zero. This scheduling class does not take a priority argument. This scheduling class is permitted for an ordinary user (since kernel 2.6.25).Best effort
This is the effective scheduling class for any process that has not asked for a specific io priority. This class takes a priority argument from 0-7, with lower number being higher priority. Programs running at the same best effort priority are served in a round-robin fashion.
source:http://linuxpoison.blogspot.com/2011/11/1357816775591.html