IONICE(1)							   User Commands							 IONICE(1)

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/.

util-linux							     July 2011								 IONICE(1)