ionice command

What’s ionice command
ionice - set or get process I/O scheduling class and priority
Ionice 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:
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).
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).
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.
Ionice example of usage:
ionice -c 3 -p 111
Sets process with PID 111 as an idle I/O process.
That's all what We want to say about ionice command.

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