Mastering the "swapon" and "swap" Linux Commands

Prepare Your Devices for Paging and File Swapping

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Swapon specifies the devices on which paging and file swapping will take place. Calls to swapon normally occur in the system multi-user initialization file /etc/rc that makes all swap devices available, so that the paging and swapping activity is interleaved across several devices and files.

Synopsis

/sbin/swapon [-h -V] 
/sbin/swapon -a [-v] [-e] 
/sbin/swapon [-v] [-p priorityspecialfile ... 
/sbin/swapon [-s] 
/sbin/swapoff [-h -V] 
/sbin/swapoff -a 
/sbin/swapoff specialfile ...  

Switches

Swapon supports several switches to extend or refine the command's execution.

-h

Provide help

-V

Display version

-s

Display swap usage summary by device. Equivalent to cat /proc/swaps. Not available before Linux 2.1.25.

-a

All devices marked as swap swap devices in /etc/fstab are made available. Devices that are already running as swap are silently skipped.

-e

When -a is used with swapon-e makes swapon silently skip devices that do not exist.

-p priority

Specify a priority for swapon. This option is only available if swapon was compiled under and is used under a 1.3.2 or later kernel.  The priority is a value between 0 and 32767. See swapon(2) for a full description of swap priorities. Add pri=value to the option field of/etc/fstab for use with swapon -a.

Swapoff disables swapping on the specified devices and files. When the -a flag is given, swapping is disabled on all known swap devices and files (as found in /proc/swaps or /etc/fstab).

Notes

You should not use swapon on a file with holes. Swap over NFS may not work. 

Related commands include:

  • swapon(2)
  • swapoff(2)
  • init(8)
  • mkswap(8)
  • rc(8)
  • mount(8) 

The specific use of swapon may vary by distribution and kernel-release level. Use the man command (% man) to see how a command is used on your particular computer.