NAME

       ionice - set or get process I/O scheduling class and priority

SYNOPSIS

       ionice [-c class] [-n level] [-t] -p PID...
       ionice [-c class] [-n level] [-t] command [argument...]

DESCRIPTION

       This  program  sets  or  gets  the I/O scheduling class and priority for a program.  If no
       arguments or just -p is given, ionice will query the  current  I/O  scheduling  class  and
       priority for that process.

       When command is given, ionice will run this command with the given arguments.  If no class
       is specified, then command will be executed with the "best-effort" scheduling class.   The
       default priority level is 4.

       As of this writing, a process can be in one of three scheduling classes:

       Idle   A  program  running  with  idle  I/O priority will only get disk time when no other
              program has asked for disk I/O for a defined grace period.  The impact of  an  idle
              I/O  process  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).

       Best-effort
              This  is  the  effective  scheduling class for any process that has not asked for a
              specific I/O priority.  This class takes a priority argument from 0-7, with a 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  I/O  priority
              formally  uses  "none"  as  scheduling class, but the I/O scheduler will treat such
              processes 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_priority = (cpu_nice + 20) / 5.

              For kernels after 2.6.26 with the CFQ I/O scheduler, a process that has  not  asked
              for an I/O priority inherits its CPU scheduling class.  The I/O priority is derived
              from the CPU nice level of the process (same as before kernel 2.6.26).

       Realtime
              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
              scheduling window.  This scheduling class is not permitted for an  ordinary  (i.e.,
              non-root) user.

OPTIONS

       -c, --class class
              Specify  the  name  or  number  of  the  scheduling class to use; 0 for none, 1 for
              realtime, 2 for best-effort, 3 for idle.

       -n, --classdata level
              Specify the scheduling class data.  This only has an effect if the class accepts an
              argument.  For realtime and best-effort, 0-7 are valid data (priority levels).

       -p, --pid PID...
              Specify the process IDs of running processes for which to get or set the scheduling
              parameters.

       -t, --ignore
              Ignore failure to set the requested priority.  If command  was  specified,  run  it
              even  in case it was not possible to set the desired scheduling priority, which can
              happen due to insufficient privileges or an old kernel version.

       -h, --help
              Display help and exit.

       -V, --version
              Display version information and exit.

EXAMPLES

       # ionice -c 3 -p 89

       Sets process with PID 89 as an idle I/O 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.

NOTES

       Linux supports I/O scheduling priorities  and  classes  since  2.6.13  with  the  CFQ  I/O
       scheduler.

AUTHORS

       Jens Axboe <jens@axboe.dk>
       Karel Zak <kzak@redhat.com>

AVAILABILITY

       The   ionice   command   is   part  of  the  util-linux  package  and  is  available  from
       ftp://ftp.kernel.org/pub/linux/utils/util-linux/.