ionice - get/set program io scheduling class and priority
ionice [[-c class] [-n classdata] [-t]] -p PID [PID]...
ionice [-c class] [-n classdata] [-t] COMMAND [ARG]...
This program sets or gets the io scheduling class and priority for a program. If no argu-
ments or just -p is given, ionice will query the current io scheduling class and priority
for that process.
As of this writing, a process can be in one of three scheduling classes:
Idle A program running with idle io priority will only get disk time when no other pro-
gram has asked for disk io for a defined grace period. The impact of idle io pro-
cesses on normal system activity should be zero. This scheduling class does not
take a priority argument. Presently, this scheduling class is permitted for an
ordinary user (since kernel 2.6.25).
This is the effective scheduling class for any process that has not asked for a
specific io priority. This class takes a priority argument from 0-7, with lower
number being higher priority. Programs running at the same best effort priority are
served in a round-robin fashion.
Note that before kernel 2.6.26 a process that has not asked for an io priority for-
mally uses "none" as scheduling class, but the io scheduler will treat such pro-
cesses as if it were in the best effort class. The priority within the best effort
class will be dynamically derived from the cpu nice level of the process: io_prior-
ity = (cpu_nice + 20) / 5.
For kernels after 2.6.26 with CFQ io scheduler a process that has not asked for an
io priority inherits CPU scheduling class. The io priority is derived from the cpu
nice level of the process (same as before kernel 2.6.26).
The RT scheduling class is given first access to the disk, regardless of what else
is going on in the system. Thus the RT class needs to be used with some care, as it
can starve other processes. As with the best effort class, 8 priority levels are
defined denoting how big a time slice a given process will receive on each schedul-
ing window. This scheduling class is not permitted for an ordinary (i.e., non-root)
The scheduling class. 0 for none, 1 for real time, 2 for best-effort, 3 for idle.
The scheduling class data. This defines the class data, if the class accepts an
argument. For real time and best-effort, 0-7 is valid data.
-p pid Pass in process PID(s) to view or change already running processes. If this argu-
ment is not given, ionice will run the listed program with the given parameters.
-t Ignore failure to set requested priority. If COMMAND or PID(s) is specified, run it
even in case it was not possible to set desired scheduling priority, what can hap-
pen due to insufficient privileges or old kernel version.
# ionice -c 3 -p 89
Sets process with PID 89 as an idle io process.
# ionice -c 2 -n 0 bash
Runs 'bash' as a best-effort program with highest priority.
# ionice -p 89 91
Prints the class and priority of the processes with PID 89 and 91.
Linux supports io scheduling priorities and classes since 2.6.13 with the CFQ io sched-
Jens Axboe <email@example.com>
The ionice command is part of the util-linux package and is available from ftp://ftp.ker-
ionice August 2005 ionice(1)