swapon, swapoff - enable/disable devices and files for paging and swapping
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 uuid.
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 the
flag is given, swapping is disabled on all known swap devices and
files (as found in /proc/swaps
- -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 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. The /etc/fstab mount
options discard, 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
- -f, --fixpgsz
- 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.
- -h, --help
- Display help text and exit.
- -L label
- 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
string is evaluated last and overrides all other command line
- -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
- -s, --summary
- 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
- Display --show output without aligning table
- 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.
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
, and on copy-on-write filesystems like btrfs
It is recommended to use dd
(1) and /dev/zero
to avoid holes on XFS
may not work correctly when using a swap file with some versions
. 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
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
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 made.
- 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