linux man page for swapon

SWAPON(8)						     Linux Programmer's Manual							 SWAPON(8)

NAME
       swapon, swapoff - enable/disable devices and files for paging and swapping

SYNOPSIS
       Get info:
	    swapon -s [-h] [-V]

       Enable/disable:
	    swapon [-d] [-f] [-p priority] [-v] specialfile...
	    swapoff [-v] specialfile...

       Enable/disable all:
	    swapon -a [-e] [-f] [-v]
	    swapoff -a [-v]

DESCRIPTION
       swapon is used to specify devices on which paging and swapping are to take place.

       The  device  or file used is given by the specialfile parameter. It may be of the form -L label or -U uuid to indicate a device by label or
       uuid.

       Calls to swapon normally occur in the system boot scripts making all swap devices available, so that the paging and  swapping  activity	is
       interleaved across several devices and files.

       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).

       -a, --all
	      All devices marked as ``swap'' in /etc/fstab are made available, except for those with the  ``noauto''  option.	Devices  that  are
	      already being used as swap are silently skipped.

       -d, --discard
	      Discard  freed  swap pages before they are reused, if the swap device supports the discard or trim operation.  This may improve per-
	      formance on some Solid State Devices, but often it does not.  The /etc/fstab mount option discard may be also used to enable discard
	      flag.

       -e, --ifexists
	      Silently skip devices that do not exist.	The /etc/fstab mount option nofail may be also used to skip non-existing device.

       -f, --fixpgsz
	      Reinitialize  (exec  /sbin/mkswap) the swap space if its page size does not match that of the the current running kernel.  mkswap(2)
	      initializes the whole device and does not check for bad blocks.

       -h, --help
	      Provide help.

       -L label
	      Use the partition that has the specified label.  (For this, access to /proc/partitions is needed.)

       -p, --priority priority
	      Specify the priority of the swap device.	priority is a value between 0 and 32767. Higher  numbers  indicate  higher  priority.  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.

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

       -U uuid
	      Use the partition that has the specified uuid.

       -v, --verbose
	      Be verbose.

       -V, --version
	      Display version.

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

       swapon  automatically detects and rewrites swap space signature with old software suspend data (e.g S1SUSPEND, S2SUSPEND, ...). The problem
       is that if we don't do it, then we get data corruption the next time an attempt at unsuspending is made.

       swapon may not work correctly when using a swap file with some versions of btrfs.  This is due to the swap file implementation in the  ker-
       nel  expecting  to  be  able to write to the file directly, without the assistance of the file system.  Since btrfs is a copy-on-write file
       system, the file location may not be static and corruption can result. Btrfs actively disallows the use of files on  its  file  systems	by
       refusing to map the file. This can be seen in the system log as "swapon: swapfile has holes." One possible workaround is to map the file to
       a loopback device. This will allow the file system to determine the mapping properly but may come with a performance impact.

SEE ALSO
       swapon(2), swapoff(2), fstab(5), init(8), mkswap(8), rc(8), mount(8)

FILES
       /dev/sd??  standard paging devices
       /etc/fstab ascii filesystem description table

HISTORY
       The swapon command appeared in 4.0BSD.

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

Linux 1.x							 25 September 1995							 SWAPON(8)