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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  avail‐
       able,  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 oper‐
              ation.  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 discard freed swap pages before they  are  reused,
              while  swapping.   If no policy is selected, the default behavior is to enable both
              discard types.   The  /etc/fstab  mount  options  discard,  discard=once,  or  dis‐
              card=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/parti‐
              tions 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 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".  Not avail‐
              able  before  Linux  2.1.25.   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.

       --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
       You should not use swapon on a file with holes.  This can be seen in the system log as

              swapon: swapfile has 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  preallocated
       files  (e.g.  fallocate(1)) on filesystems like XFS or ext4, and on copy-on-write filesys‐
       tems like btrfs.

       It is recommended to use dd(1) and /dev/zero to avoid holes on XFS and ext4.

       swapon may not work correctly when using a swap file with some versions of btrfs.  This is
       due  to  btrfs  being  a copy-on-write filesystem: the file location may not be static and
       corruption can result.  Btrfs actively disallows the use of swap files on its  filesystems
       by refusing to map the file.

       One possible workaround is to map the swap file to a loopback device.  This will allow the
       filesystem to determine the mapping properly but may come with a performance impact.

       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.

ENVIRONMENT
       LIBMOUNT_DEBUG=all
              enables libmount debug output.

       LIBBLKID_DEBUG=all
              enables libblkid debug output.

SEE ALSO
       swapon(2), swapoff(2), fstab(5), init(8), mkswap(8), rc(8), mount(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  ftp://ftp.ker‐
       nel.org/pub/linux/utils/util-linux/.

util-linux                                 October 2014                                 SWAPON(8)

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