If You running IONICE on a Linux VPS, you may have an issue. At least on an OpenVZ VPS im unable to set process disk priority using ionice.

/bin/nice -n 19 /usr/bin/ionice -c2 -n7 echo test
ionice: ioprio_set failed: Operation not permitted
The OpenVZ server admin has rights to change whole VPS processes HDD I/O priority like this i think:
sudo vzctl set 100 --ioprio 0 --save
sudo vzctl set 100 --ioprio 7 --save
100 is openvz VPS id, ioprio 0 is giving lowest priority to VPS100 processes amongst other VPSs, 7 is highest priority (as explained here)

Instead of ionice, one can use nice

here are nice and ionice usage examples: http://internetlifeforum.com/beginners/2283-how-run-linux-command-script-lowest-priority-nice-ionice