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

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, dis‐
              card=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.)

       -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.

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.

ENVIRONMENT
       LIBMOUNT_DEBUG=all
              enables libmount debug output.

       LIBBLKID_DEBUG=all
              enables libblkid debug output.


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

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

HISTORY
       The swapon command appeared in 4.0BSD.

AVAILABILITY
       The swapon command is part of the util-linux package and is available from https://www.kernel.org/pub/linux/utils/util-linux/.



util-linux                                                                                       October 2014                                                                                       SWAPON(8)```