Section: System Administration (8)
Updated: October 2014
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.
Output all 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.
has the following exit status values since v2.36:
system has insufficient memory to stop swapping (OOM)
swapoff syscall failed for another reason
non-swapoff syscall system error (out of memory, ...)
usage or syntax error
all swapoff failed on --all
some swapoff succeeded on --all
The command swapoff --all returns 0 (all succeeded), 32 (all failed), or 64 (some
failed, some succeeded).
The old versions before v2.36 has no documented exit status, 0 means success in all versions.
enables libmount debug output.
enables libblkid debug output.
standard paging devices
ascii filesystem description table
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 filesystems like Btrfs.
create files with holes. These files will be rejected by swapon.
Preallocated files created by
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
Swap files on Btrfs are supported since Linux 5.0 on files with nocow attribute.
manual page for more details.
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
command appeared in 4.0BSD.
The swapon command is part of the util-linux package and is available from