OPTIONS
       -c, --class class
              Specify the name or number of the scheduling class to use; 
              0 for none, 1 for realtime, 2 for best-effort, 3 for idle.

       -n, --classdata level
              Specify the scheduling class data.  This only has an effect if the class accepts an argument.
              For  real-time and best-effort, 0-7 are valid data (priority levels).

# nice -n 19 ionice -c 2 -n 7 COMMAND

# ionice -p 27367
unknown: prio 4

# ionice -c 3 -p 27367
# ionice -p 27367
idle
#

# ionice -c 2 -p 27367
# ionice -p 27367
best-effort: prio 4
#