SWAPON(8) System Administration SWAPON(8)
swapon, swapoff - enable/disable devices and files for paging and swap-
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
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 asyn-
chronously 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 dis-
card, discard=once, or discard=pages may also be used to enable
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) ini-
tializes 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.)
-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 com-
mand 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.
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.
--raw Display --show output without aligning table columns.
Display swap size in bytes in --show output instead of in user-
Use the partition that has the specified uuid.
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
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.
enables libmount debug output.
enables libblkid debug output.
swapoff(2), swapon(2), fstab(5), init(8), 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
util-linux October 2014 SWAPON(8)