swapon, swapoff — enable/disable devices and files for paging and swapping
swapon [options] [
−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
−U uuid to indicate a device by label
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.
disables swapping on the specified devices and files. When
−a flag is given,
swapping is disabled on all known swap devices and files (as
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:
to perform a single-time discard operation for the
whole swap area at swapon; or
asynchronously discard freed swap pages before they are
avaliable for reuse. If no policy is selected, the
default behavior is to enable both discard types. The
/etc/fstab mount options
discard=pages may also
be used to enable discard flags.
Silently skip devices that do not exist. The
/etc/fstab mount option
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,
/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.
is a value between −1 and 32767. Higher numbers
indicate higher priority. See swapon(2) for a full
description of swap priorities. Add
value to the option
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
provides better control on output data.
Display a definable table of swap areas. See the
for a list of available columns.
Do not print headings when displaying
−−show output without
aligning table columns.
Display swap size in bytes in
−−show output instead of in
Use the partition that has the specified
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.
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.
NFS may not
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.
The swapon command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
Copyright (c) 1980, 1991 Regents of the University of California.
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must display the following acknowledgement:
This product includes software developed by the University of
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(#)swapon.8 6.3 (Berkeley) 3/16/91