Provided by: mount_2.20.1-5.1ubuntu20.9_amd64 
NAME
swapon, swapoff - enable/disable devices and files for paging and swapping
SYNOPSIS
Get info:
swapon -s [-h] [-V]
Enable/disable:
swapon [-d] [-f] [-p priority] [-v] specialfile...
swapoff [-v] specialfile...
Enable/disable all:
swapon -a [-e] [-f] [-v]
swapoff -a [-v]
DESCRIPTION
swapon is used to specify devices on which paging and swapping are to take place.
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.
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).
-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
Discard freed swap pages before they are reused, if the swap device supports the discard or trim
operation. This may improve performance on some Solid State Devices, but often it does not. The
/etc/fstab mount option discard may be also used to enable discard flag.
-e, --ifexists
Silently skip devices that do not exist. The /etc/fstab mount option nofail may be also 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 the
current running kernel. mkswap(2) initializes the whole device and does not check for bad blocks.
-h, --help
Provide help.
-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 0 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.
-s, --summary
Display swap usage summary by device. Equivalent to "cat /proc/swaps". Not available before Linux
2.1.25.
-U uuid
Use the partition that has the specified uuid.
-v, --verbose
Be verbose.
-V, --version
Display version.
NOTES
You should not use swapon on a file with holes. Swap over NFS may not work.
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
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.
SEE ALSO
swapon(2), swapoff(2), fstab(5), init(8), mkswap(8), rc(8), mount(8)
FILES
/dev/sd?? standard paging devices
/etc/fstab ascii filesystem description table
HISTORY
The swapon command appeared in 4.0BSD.
AVAILABILITY
The swapon command is part of the util-linux package and is available from
ftp://ftp.kernel.org/pub/linux/utils/util-linux/.