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 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 asynchronously discard freed
              swap pages before they are available for reuse.  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 command 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".  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 refus-
       ing 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 prob-
       lem 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

       swapoff(2), swapon(2), fstab(5), init(8), mkswap(8), mount(8), rc(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 https://www.kernel.org/pub/linux/utils/util-linux/.

util-linux                                                         October 2014                                                          SWAPON(8)