| swapoff(8) - phpMan
SWAPON(8) System Administration SWAPON(8)
NAME
swapon, swapoff - enable/disable devices and files for paging and swapping
SYNOPSIS
swapon [options] [specialfile...]
swapoff [-va] [specialfile...]
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 avail‐
able, 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).
OPTIONS
-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 oper‐
ation. 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 dis‐
card=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/parti‐
tions 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.
-s, --summary
Display swap usage summary by device. Equivalent to "cat /proc/swaps". Not avail‐
able 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.
--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 in user-friendly units.
-U uuid
Use the partition that has the specified uuid.
-v, --verbose
Be verbose.
-V, --version
Display version information and exit.
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.
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 July 2014 SWAPON(8)
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