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 skipped.

       -T, --fstab path
           Specifies an alternative fstab file for compatibility with
           mount(8). If path is a directory, then the files in the
           directory are sorted by strverscmp(3); files that start with
           "." or without an .fstab extension are ignored. The option
           can be specified more than once. This option is mostly
           designed for initramfs or chroot scripts where additional
           configuration is specified beyond standard system

       -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

               to perform a single-time discard operation for the whole
               swap area at swapon; or

               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

       -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 0 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, Linux kernel defaults
           to negative numbers.

       -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.

           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.

       -h, --help
           Display help text and exit.

       -V, --version
           Print version and exit.

EXIT STATUS         top

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


           system has insufficient memory to stop swapping (OOM)

           swapoff(2) syscall failed for another reason

           non-swapoff(2) syscall system error (out of memory, ...)

           usage or syntax error

           all swapoff failed on --all

           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

           standard paging devices

           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.

       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)

REPORTING BUGS         top

       For bug reports, use the issue tracker at

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
       2024-06-14. (At that time, the date of the most recent commit
       that was found in the repository was 2024-06-10.) 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 2.39.594-1e0ad      2023-08-25                      SWAPON(8)

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