How to give a user ionice level permission? [closed]
This error (EPERM) indicates that you don't have sufficient privileges to perform that operation. ioprio_set():
An unprivileged process may only set the I/O priority of a process whose real UID matches the real or effective UID of the calling process. A process which has the CAP_SYS_NICE capability can change the priority of any process.
So, either become the superuser (use sudo), or use ionice on one of your own processes.