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.

       -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 configuration.

       -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.

       -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.

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

       -V, --version
           Print version 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(2) syscall failed for another reason

       8
           non-swapoff(2) 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/util-linux/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/>.