How disk IO priority is related with process priority? I'm running software that overloads disk IO sometimes. I don't need fast response from that software, I need fast response from other applications, so I could set low process priority for that. I want to ask how process priority affects disk IO priority for a process. I tried a small experiment: I set low priority (in System Monitor under GNOME) for a process and checked IO priority with `ionice`. **Result:** IO priority = 0 for normal process priority IO priority = 4 for low process priority But will this always work like this? Is IO priority always reduced when I reduce process priority?

Under Linux, by default, a process's IO priority is derived from its CPU priority according to the formula


io_priority = (cpu_nice + 20) / 5


IO priority ranges from 0 to 7 with 0 being the highest priority. CPU niceness ranges from -20 to 19 with -20 being the highest priority.

You can use the `ionice` command to change a process's IO priority. If you want that process to run only when the system isn't otherwise busy, make it run under the “idle” class rather than the default “best-effort” class:


ionice -c 3 -p $PID
ionice -c 3 mycommand --someoption


Even with the lowest priority, a disk-intensive process tends to slow the system down, if nothing else because it pollutes the cache.

See the `ionice` man page for more information.