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".   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
       filesystems 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/.