SWAPON(8) System Administration SWAPON(8)
swapon, swapoff - enable/disable devices and files for paging and swap-
swapon [options] [specialfile...]
swapoff [-va] [specialfile...]
swapon 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
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.
swapoff disables swapping on the specified devices and files. When the
-a flag is given, swapping is disabled on all known swap devices and
files (as found in /proc/swaps or /etc/fstab).
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:
--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 dis-
card types. The /etc/fstab mount options discard, discard=once,
or discard=pages may also be used to enable discard flags.
Silently skip devices that do not exist. The /etc/fstab mount
option nofail may also be used to skip non-existing device.
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.
Display help text and exit.
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 -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 --show that provides better
control on output data.
--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.
--raw Display --show output without aligning table columns.
Display swap size in bytes in --show output instead of in user-
Use the partition that has the specified uuid.
Display version information and exit.
You should not use swapon on a file with holes. Swap over NFS may not
swapon 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.
swapon 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 assis-
tance 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 prop-
erly but may come with a performance impact.
enables debug output.
swapon(2), swapoff(2), fstab(5), init(8), mkswap(8), rc(8), mount(8)
/dev/sd?? standard paging devices
/etc/fstab ascii filesystem description table
The swapon command appeared in 4.0BSD.
The swapon command is part of the util-linux package and is available
util-linux July 2014 SWAPON(8)