Ubuntu Manpages

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 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).

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.
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 blocks.
Display help text and exit.
Use the partition that has the specified label. (For this, access to /proc/partitions is needed.)
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.
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". 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 user-friendly units.
Use the partition that has the specified uuid.
Be verbose.
Display version information and exit.

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.

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.

enables libmount debug output.
enables libblkid debug output.

swapoff(2), swapon(2), fstab(5), init(8), fallocate(1), mkswap(8), mount(8), rc(8)

/dev/sd?? standard paging devices
/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 https://www.kernel.org/pub/linux/utils/util-linux/.