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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  avail-
       able,  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  performance 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/parti-
	      tions 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  avail-
	      able 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 kernel 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.ker-
       nel.org/pub/linux/utils/util-linux/.

Linux 1.x				25 September 1995				SWAPON(8)
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