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  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
              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.)

       -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  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)