SWAPON(8) System Administration SWAPON(8)
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 pag‐
ing 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 [=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 be also used to enable
discard flags.
-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 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.)
--noheadings
Do not print headings when displaying --show output.
--raw Display --show output without aligning table columns.
--bytes
Display swap size in bytes in --show output instead of user friendly size and unit. -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 cor‐
ruption 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.
ENVIRONMENT
LIBMOUNT_DEBUG=0xffff
enables debug output.
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.ker‐
nel.org/pub/linux/utils/util-linux/.
util-linux September 1995 SWAPON(8)
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