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(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
       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
       LIBMOUNT_DEBUG=all
           enables libmount debug output.

       LIBBLKID_DEBUG=all
           enables libblkid debug output.

FILES
       /dev/sd??
           standard paging devices

       /etc/fstab
           ascii filesystem description table

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.

HISTORY
       The swapon command appeared in 4.0BSD.

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

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

AVAILABILITY
       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/>.

util-linux 2.37.2                 2021-06-02                         SWAPON(8)