Section: System Administration (8)
Updated: October 2014
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swapon, swapoff - enable/disable devices and files for paging and swapping
is used to specify devices on which paging and swapping are to take place.
The device or file used is given by the
parameter. It may be of the form
to indicate a device by label or uuid.
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.
flag is given, swapping is disabled on all known swap devices and files
(as found in
- -a, --all
All devices marked as ``swap'' in
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:
to perform a single-time discard operation for the whole swap area at swapon;
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.
may also be used to enable discard flags.
- -e, --ifexists
Silently skip devices that do not exist.
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.
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
(For this, access to
- -o, --options opts
Specify swap options by an fstab-compatible comma-separated string.
The opts string is evaluated last and overrides all other
command line options.
swapon -o pri=1,discard=pages,nofail /dev/sda2
- -p, --priority priority
Specify the priority of the swap device.
is a value between -1 and 32767. Higher numbers indicate
higher priority. See
for a full description of swap priorities. Add
to the option field of
for use with
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.
Display a definable table of swap areas. See the
output for a list of available columns.
Do not print headings when displaying
output without aligning table columns.
Display swap size in bytes in
output instead of in user-friendly units.
- -U uuid
Use the partition that has the specified
- -v, --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
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.
on filesystems like XFS or ext4, and on copy-on-write
filesystems like btrfs.
swapon: swapfile has holes.
It is recommended to use
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 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.
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
enables libmount debug output.
enables libblkid debug output.
standard paging devices
ascii filesystem description table
command appeared in 4.0BSD.
The swapon command is part of the util-linux package and is available from
- SEE ALSO
This document was created by
using the manual pages.
Time: 15:17:50 GMT, September 26, 2018