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)