swapon(8) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | EXIT STATUS | ENVIRONMENT | FILES | NOTES | HISTORY | SEE ALSO | REPORTING BUGS | AVAILABILITY

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 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.

       --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.

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

       LIBMOUNT_DEBUG=all
           enables libmount debug output.

       LIBBLKID_DEBUG=all
           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 /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.

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)

REPORTING BUGS         top

       For bug reports, use the issue tracker at
       https://github.com/karelzak/util-linux/issues.

AVAILABILITY         top

       The swapon command is part of the util-linux package which can be
       downloaded from Linux Kernel Archive
       <https://www.kernel.org/pub/linux/utils/util-linux/>. 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
       2021-06-20. (At that time, the date of the most recent commit
       that was found in the repository was 2021-06-18.) 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 man-pages@man7.org

util-linux 2.37.85-637cc       2021-04-02                      SWAPON(8)

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