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After running
ionice -c2 -n7 <PID> on an I/O-intensive process (VirtualBox VM) and checking the result with ionice -p <PID> it shows best-effort: prio 7 which is expected.

But when monitoring the overall disk I/O of all processes via iotop it shows in PRIO column a value of be/4 for that process, which I assume means best-effort and level (priority) 4, which is average priority and not expected.

Can somebody comment why there's a difference between what ionice and what iotop displays? And is there another way to verify the actual I/O priority of a given process other then with ionice -p ?

From what I understood, there's /proc/<pid>/io but it only displays bytes/characters read and written, but not the priority, cf. Know which process does I/O without iotop
Also How do I view the IO priority of a process? only lists ionice -p

For completeness sake, this is on RHEL 6.7 with iotop 0.3.2, and the scheduler is the default CFQ.

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Seems I already found part of the answer. By default iotop shows all processes and threads, and I usually run it via iotop -o which only shows threads and processes doing I/O. The VirtualBox process shown as be/4 is actually a thread, and when I run iotop -p <pid> against the PID which I changed via ionice then it correctly displays be/7 which is expected.

Would still be interesting to know why one thread of that process has a different I/O class/priority, but my ionice does not have -P option to change the I/O class/priority of a process group. Would be nice if someone knows a workaround/solution for that.

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