SWAPON(8)		     System Administration		     SWAPON(8)



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

SYNOPSIS
       swapon [options] [specialfile...]
       swapoff [-va] [specialfile...]

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).


OPTIONS
       -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[=policy]
	      Enable  swap  discards,  if the swap backing device supports the
	      discard or trim operation.  This may improve performance on some
	      Solid  State  Devices, but often it does not.  The option allows
	      one to select  between  two  available  swap  discard  policies:
	      --discard=once  to  perform  a single-time discard operation for
	      the whole swap area  at  swapon;	or  --discard=pages  to	 asyn-
	      chronously  discard  freed  swap pages before they are available
	      for reuse.  If no policy is selected, the default behavior is to
	      enable  both  discard  types.  The /etc/fstab mount options dis-
	      card, discard=once, or discard=pages may also be used to	enable
	      discard flags.

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


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

       -h, --help
	      Display help text and exit.

       -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 -1 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.  When no priority is defined, it defaults to -1.

       -s, --summary
	      Display  swap  usage  summary  by	 device.   Equivalent  to "cat
	      /proc/swaps".  This output format is  DEPRECATED	in  favour  of
	      --show that provides better control on output data.

       --show[=column...]
	      Display  a definable table of swap areas.	 See the --help output
	      for a list of available columns.

       --output-all
	      Output all available columns.

       --noheadings
	      Do not print headings when displaying --show output.

       --raw  Display --show output without aligning table columns.

       --bytes
	      Display swap size in bytes in --show output instead of in	 user-
	      friendly units.

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

       -v, --verbose
	      Be verbose.

       -V, --version
	      Display version information and exit.

NOTES
   Files with holes
       The  swap file implementation in the kernel expects to be able to write
       to the file directly, without the assistance of the  filesystem.	  This
       is  a problem on files with holes or on copy-on-write files on filesys-
       tems like Btrfs.

       Commands like cp(1) or truncate(1)  create  files  with	holes.	 These
       files will be rejected by swapon.

       Preallocated  files created by fallocate(1) may be interpreted as files
       with holes too depending of the filesystem.   Preallocated  swap	 files
       are supported on XFS since Linux 4.18.

       The  most  portable  solution to create a swap file is to use dd(1) and
       /dev/zero.

   Btrfs
       Swap files on Btrfs are supported since Linux 5.0 on files  with	 nocow
       attribute.  See the btrfs(5) manual page for more details.

   NFS
       Swap over NFS may not work.

   Suspend
       swapon  automatically  detects and rewrites a 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.

ENVIRONMENT
       LIBMOUNT_DEBUG=all
	      enables libmount debug output.

       LIBBLKID_DEBUG=all
	      enables libblkid debug output.


SEE ALSO
       swapoff(2),  swapon(2),	fstab(5),  init(8),  fallocate(1),  mkswap(8),
       mount(8), rc(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 https://www.kernel.org/pub/linux/utils/util-linux/.



util-linux			 October 2014			     SWAPON(8)