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
              Discard  freed  swap  pages  before they are reused, if the swap
              device supports the discard or trim operation.  This may improve
              performance  on some Solid State Devices, but often it does not.
              The /etc/fstab mount option discard may be also used  to  enable
              discard flag.

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

       -f, --fixpgsz
              Reinitialize (exec /sbin/mkswap) the swap space if its page size
              does   not  match  that  of  the  the  current  running  kernel.
              mkswap(2) initializes the whole device and does  not  check  for
              bad blocks.


       -s, --summary
              Display  swap  usage  summary  by  device.  Equivalent  to  "cat
              /proc/swaps".  Not available before Linux 2.1.25.

       -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.

       swapon(2), swapoff(2), fstab(5), init(8), mkswap(8), rc(8), mount(8)

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

       The swapon command appeared in 4.0BSD.

       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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