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       Get info:
            swapon -s [-h] [-V]

            swapon [-d] [-f] [-p priority] [-v] specialfile...
            swapoff [-v] specialfile...

       Enable/disable all:
            swapon -a [-e] [-f] [-v]
            swapoff -a [-v]


       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

       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).

       -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  discard
              freed  swap pages before they are reused, while swapping.  If no
              policy is selected, the default behavior is to enable both  dis-
              card types.  The /etc/fstab mount options discard, discard=once,
              or discard=pages may be also used to enable discard flags.

       -e, --ifexists
              Silently skip devices that do not exist.  The  /etc/fstab  mount
              option nofail may be also used to skip non-existing device.

              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 priority is not 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.

       --show [column,column]
              Display definable device table similar to --summary output.  See
              --help output for column list.

              Do not print headings when displaying --show output.

       --raw  Display --show output without aligning table columns.

              Display swap size in bytes in  --show  output  instead  of  user
              friendly  size and unit.  -U uuid Use the partition that has the
              specified uuid.

       -v, --verbose
              Be verbose.

       -V, --version
              Display version.


       You should not use swapon on a file with holes.  Swap over NFS may  not

       swapon automatically detects and rewrites 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.

       swapon may not work correctly when using a swap file with some versions
       of  btrfs.   This  is due to the swap file implementation in the kernel
       expecting to be able to write to the file directly, without the  assis-
       tance  of the file system.  Since btrfs is a copy-on-write file system,
       the file location may not be static and corruption  can  result.  Btrfs
       actively  disallows the use of files on its file systems by refusing to
       map the file. This can be seen in the system log as  "swapon:  swapfile
       has  holes."  One  possible workaround is to map the file to a loopback
       device. This will allow the file system to determine the mapping  prop-
       erly but may come with a performance impact.


       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 September 1995 SWAPON(8)

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