swapon(8) — Linux manual page


SWAPON(8)                   System Administration                  SWAPON(8)

NAME         top

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

SYNOPSIS         top

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

DESCRIPTION         top

       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         top

       -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

       -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

       -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.

EXIT STATUS         top

       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.

ENVIRONMENT         top

              enables libmount debug output.

              enables libblkid debug output.

FILES         top

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

NOTES         top

   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.

HISTORY         top

       The swapon command appeared in 4.0BSD.

SEE ALSO         top

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

AVAILABILITY         top

       The swapon command is part of the util-linux package and is available
       from https://www.kernel.org/pub/linux/utils/util-linux/.

COLOPHON         top

       This page is part of the util-linux (a random collection of Linux
       utilities) project.  Information about the project can be found at 
       ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩.  If you have a
       bug report for this manual page, send it to
       util-linux@vger.kernel.org.  This page was obtained from the
       project's upstream Git repository
       ⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
       2020-09-18.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-09-15.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

util-linux                      October 2014                       SWAPON(8)

Pages that refer to this page: swapoff(2)swapon(2)fstab(5)org.freedesktop.systemd1(5)proc(5)procfs(5)systemd.swap(5)mkswap(8)mount(8)swaplabel(8)