The options available are :

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.

  - Best effort
  This is the default scheduling class for any process that hasn't asked for a specific io priority. Programs running at the same best effort priority are served in a round-robin fashion.
  - Real time
  The RT scheduling class is given first access to the disk, regardless of what else is going on in the system. Thus the RT class needs to be used with some care, as it can starve other processes.