swapoff(8)



SWAPON(8)                    System Administration                   SWAPON(8)

NAME
       swapon, swapoff - enable/disable devices and files for paging and swap-
       ping

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  asyn-
              chronously  discard  freed  swap pages before they are avaliable
              for reuse.  If no policy is selected, the default behavior is to
              enable  both  discard  types.  The /etc/fstab mount options dis-
              card, 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(2) ini-
              tializes 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  com-
              mand 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".  Not available 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 filesystems 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.kernel.org/pub/linux/utils/util-linux/.

util-linux                       October 2014                        SWAPON(8)

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