SWAPON(8)                    System Administration                   SWAPON(8)

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

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

       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

       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).

       -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(8)
              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.

              Display a definable table of swap areas.  See the --help output
              for a list of available columns.

              Output all available columns.

              Do not print headings when displaying --show output.

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

              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.

       swapoff has the following exit status values since v2.36:

       0      success

       2      system has insufficient memory to stop swapping (OOM)

       4      swapoff syscall failed for another reason

       8      non-swapoff syscall system error (out of memory, ...)

       16     usage or syntax error

       32     all swapoff failed on --all

       64     some swapoff succeeded on --all

              The command swapoff --all returns 0 (all succeeded), 32 (all
              failed), or 64 (some failed, some succeeded).

              The old versions before v2.36 has no documented exit status, 0
              means success in all versions.

              enables libmount debug output.

              enables libblkid debug output.

       /dev/sd??  standard paging devices
       /etc/fstab ascii filesystem description table

   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

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

       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.

       The swapon command appeared in 4.0BSD.

       swapoff(2), swapon(2), fstab(5), init(8), fallocate(1), mkswap(8),
       mount(8), rc(8)

       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)