− enable/disable devices and files for paging and
swapoff [−va] [specialfile...]
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
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
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 discard freed swap
pages before they are reused, while swapping. 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:
string is evaluated last and overrides all other
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.
Display −−show output without
aligning table columns.
Display swap size in bytes in
−−show output instead of in user-friendly
Use the partition that has the
Display version information and
You should not
use swapon on a file with holes. This can be seen in
the system log as
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
recommended to use dd(1) and /dev/zero to
avoid holes on XFS and ext4.
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
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.
NFS may not work.
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
enables libmount debug
enables libblkid debug
swapoff(2), fstab(5), init(8),
mkswap(8), rc(8), mount(8)
standard paging devices
/etc/fstab ascii filesystem description table
swapon command appeared in 4.0BSD.
command is part of the util-linux package and is available