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).
- -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.
- --output-all
- Output all 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.
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
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.
/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/.