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 discard freed swap pages before they are reused,
while swapping. 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 non-existing device.
- -f, --fixpgsz
- Reinitialize (exec /sbin/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.)
- -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 priority is not 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
- --show [column, ...]
- Display definable device table similar to --summary output. See
--help output for a list of available columns.
- Do not print headings when displaying --show output.
- Display --show output without aligning table columns.
- Display swap size in bytes in --show output instead of in
- -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. Swap over NFS may not
automatically detects and rewrites 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.
may not work correctly when using a swap file with some versions
of btrfs. This is due to the swap file implementation in the kernel expecting
to be able to write to the file directly, without the assistance of the file
system. Since btrfs is a copy-on-write file system, the file location may not
be static and corruption can result. Btrfs actively disallows the use of files
on its file systems by refusing to map the file. This can be seen in the
system log as "swapon: swapfile has holes." One possible workaround
is to map the file to a loopback device. This will allow the file system to
determine the mapping properly but may come with a performance impact.
- enables 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