swapon, swapoff - enable/disable devices and files for paging and swapping
swapon [options] [specialfile...]
swapoff [-va] [specialfile...]
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 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
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
files with holes or on copy-on-write files on filesystems like Btrfs.
- -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.)
- -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
- -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.
- Display a definable table of swap areas. See the --help output for
a list of available columns.
- Output all available columns.
- Do not print headings when displaying --show output.
- Display --show output without aligning table columns.
- Display swap size in bytes in --show output instead of in
- -U uuid
- Use the partition that has the specified uuid.
- -v, --verbose
- Be verbose.
- -V, --version
- Display version information and exit.
Commands like cp(1) or truncate(1) create files with
holes. These files will be rejected by swapon.
Preallocated files created by fallocate(1) may be
interpreted as files with holes too depending of the filesystem.
Preallocated swap files are supported on XFS since Linux 4.18.
The most portable solution to create a swap file is to use
dd(1) and /dev/zero.
Swap files on Btrfs are supported since Linux 5.0 on files with nocow attribute.
See the btrfs(5) manual page for more details.
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.
swapoff(2), swapon(2), fstab(5), init(8),
fallocate(1), mkswap(8), mount(8), rc(8)
/dev/sd?? standard paging devices
- enables libmount debug output.
- enables libblkid debug output.
/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
Visit the GSP FreeBSD Man Page Interface.
Output converted with ManDoc.