swapon is used to specify devices on which paging and swapping are to
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
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).
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
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.
Silently skip devices that do not exist. The /etc/fstab mount
option nofail may also be used to skip non-existing device.
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
Display help text and exit.
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.
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.
Display a definable table of swap areas. See the --help
output for a list of available columns.
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 in
Use the partition that has the specified uuid.
Display version information and exit.
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
It is recommended to use dd(1) and /dev/zero to avoid holes on XFS
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.
This page is part of the util-linux (a random collection of Linux
utilities) project. Information about the project can be found at
⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have a
bug report for this manual page, send it to
email@example.com. This page was obtained from the
project's upstream Git repository
2016-08-07. If you discover any rendering problems in this HTML ver‐
sion of the page, or you believe there is a better or more up-to-date
source for the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original man‐
ual page), send a mail to firstname.lastname@example.org
util-linux October 2014 SWAPON(8)