NAME | SYNOPSIS | DESCRIPTION | OPTIONS | NOTES | ENVIRONMENT | SEE ALSO | FILES | HISTORY | AVAILABILITY | COLOPHON

SWAPON(8)                   System Administration                  SWAPON(8)

NAME         top

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

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
              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 avaliable 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         top

       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         top

       LIBMOUNT_DEBUG=all
              enables libmount debug output.

       LIBBLKID_DEBUG=all
              enables libblkid debug output.

SEE ALSO         top

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

FILES         top

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

HISTORY         top

       The swapon command appeared in 4.0BSD.

AVAILABILITY         top

       The swapon command is part of the util-linux package and is available
       from ftp://ftp.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
       2016-05-10.  If you discover any rendering problems in this HTML ver‐
       sion 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 man‐
       ual page), send a mail to man-pages@man7.org

util-linux                      October 2014                       SWAPON(8)