ubuntu@ip-10-12-251-11:~$ sudo ionice -c3 -p $(pidof opennms)
ionice: option requires an argument -- 'p'

ionice - sets or gets process io scheduling class and priority.

Usage:
  ionice [OPTION] -p PID [PID...]
  ionice [OPTION] COMMAND

Options:
  -c, --class <class>   scheduling class name or number
                           0: none, 1: realtime, 2: best-effort, 3: idle
  -n, --classdata <num> scheduling class data
                           0-7 for realtime and best-effort classes
  -p, --pid=PID         view or modify already running process
  -t, --ignore          ignore failures
  -V, --version         output version information and exit
  -h, --help            display this help and exit

sudo rrenice 19 opennms

sudo renice -n 19 -p $(pidof opennms)

ionice -c3 -p $(pidof opennms)