SWAPON(8) System Administration SWAPON(8)
NAME
swapon, swapoff - enable/disable devices and files for paging and swap-
ping
SYNOPSIS
swapon [options] [specialfile...]
swapoff [-va] [specialfile...]
DESCRIPTION
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 in-
terleaved 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).
OPTIONS
-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 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
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(8) ini-
tializes 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, ac-
cess 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.
-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.
NOTES
Files with 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 files with holes or on copy-on-write files on filesys-
tems 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.
Btrfs
Swap files on Btrfs are supported since Linux 5.0 on files with nocow
attribute. See the btrfs(5) manual page for more details.
NFS
Swap over NFS may not work.
Suspend
swapon automatically detects and rewrites a swap space signature with
old software suspend data (e.g., S1SUSPEND, S2SUSPEND, ...). The prob-
lem is that if we don't do it, then we get data corruption the next
time an attempt at unsuspending is made.
ENVIRONMENT
LIBMOUNT_DEBUG=all
enables libmount debug output.
LIBBLKID_DEBUG=all
enables libblkid debug output.
SEE ALSO
swapoff(2), swapon(2), fstab(5), init(8), fallocate(1), mkswap(8),
mount(8), rc(8)
FILES
/dev/sd?? standard paging devices
/etc/fstab ascii filesystem description table
HISTORY
The swapon command appeared in 4.0BSD.
AVAILABILITY
The swapon command is part of the util-linux package and is available
from https://www.kernel.org/pub/linux/utils/util-linux/.
util-linux October 2014 SWAPON(8)