swapon(8)

NAME

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

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
          asynchronously   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 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)
          initializes 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
          command line 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/.

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