swapon

SWAPON(8)                    System Administration                   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".  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/.



util-linux                       October 2014                        SWAPON(8)