swapon(1), swapoff(1) - enable/disable devices and files for paging and swapping

[giantpanda]

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  discard
	      freed  swap pages before they are reused, while swapping.  If no
	      policy is selected, the default behavior is to enable both  dis‐
	      card types.  The /etc/fstab mount options discard, 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.)

       -o, --options opts
	      Specify swap  options  by  an  fstab-compatible  comma-separated
	      string.  For example:

		     swapon -o pri=1,discard=pages,nofail /dev/sda2

	      The  opts  string  is  evaluated	last  and  overrides all other
	      options.

       -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".   Not  available before Linux 2.1.25.  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.

       --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
       You  should  not  use swapon on a file with holes.  This can be seen in
       the system log as

	      swapon: swapfile has 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 preallocated files (e.g.  fallocate(1)) on  filesystems
       like XFS or ext4, and on copy-on-write filesystems like btrfs.

       It  is recommended to use dd(1) and /dev/zero to avoid holes on XFS and
       ext4.

       swapon may not work correctly when using a swap file with some versions
       of  btrfs.   This is due to btrfs being a copy-on-write filesystem: the
       file location may not be  static  and  corruption  can  result.	 Btrfs
       actively disallows the use of swap files on its filesystems by refusing
       to map the file.

       One possible workaround is to map the swap file to a  loopback  device.
       This  will  allow  the filesystem to determine the mapping properly but
       may come with a performance impact.

       Swap over NFS may not work.

       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
       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/.

util-linux			 October 2014			     SWAPON(8)