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 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).
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 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 asynchronously discard freed swap pages before they are
available for reuse.
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 mkswap) the swap space if its page size does not
match that of the current running kernel. mkswap(8) initializes the
whole device and does not check for bad blocks.
-L label
Use the partition that has the specified label. (For this, access
to /proc/partitions is needed.)
-o, --options opts
Specify swap options by an fstab-compatible comma-separated string.
For example:
swapon -o pri=1,discard=pages,nofail /dev/sda2
The opts string is evaluated last and overrides all other command
line options.
-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 no
priority is defined, it defaults to -1.
-s, --summary
Display swap usage summary by device. Equivalent to cat
/proc/swaps. This output format is DEPRECATED in favour of --show
that provides better control on output data.
--show[=column...]
Display a definable table of swap areas. See the --help output for
a list of available columns.
--output-all
Output all 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.
-h, --help
Display help text and exit.
-V, --version
Print version and exit.
EXIT STATUS
swapoff has the following exit status values since v2.36:
0
success
2
system has insufficient memory to stop swapping (OOM)
4
swapoff(2) syscall failed for another reason
8
non-swapoff(2) syscall system error (out of memory, ...)
16
usage or syntax error
32
all swapoff failed on --all
64
some swapoff succeeded on --all
The command swapoff --all returns 0 (all succeeded), 32 (all failed),
or 64 (some failed, some succeeded).
+ The old versions before v2.36 has no documented exit status, 0 means
success in all versions.
ENVIRONMENT
LIBMOUNT_DEBUG=all
enables libmount debug output.
LIBBLKID_DEBUG=all
enables libblkid debug output.
FILES
/dev/sd??
standard paging devices
/etc/fstab
ascii filesystem description table
NOTES
Files with holes
The swap file implementation in the kernel expects to be able to write
to the file directly, without the assistance of the filesystem. This is
a problem on files with holes or on copy-on-write files on filesystems
like Btrfs.
Commands like cp(1) or truncate(1) create files with holes. These files
will be rejected by swapon.
Preallocated files created by fallocate(1) may be interpreted as files
with holes too depending of the filesystem. Preallocated swap files are
supported on XFS since Linux 4.18.
The most portable solution to create a swap file is to use dd(1) and
/dev/zero.
Btrfs
Swap files on Btrfs are supported since Linux 5.0 on files with nocow
attribute. See the btrfs(5) manual page for more details.
NFS
Swap over NFS may not work.
Suspend
swapon automatically detects and rewrites a 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.
HISTORY
The swapon command appeared in 4.0BSD.
SEE ALSO
swapoff(2), swapon(2), fstab(5), init(8), fallocate(1), mkswap(8),
mount(8), rc(8)
REPORTING BUGS
For bug reports, use the issue tracker at
https://github.com/util-linux/util-linux/issues.
AVAILABILITY
The swapon command is part of the util-linux package which can be
downloaded from Linux Kernel Archive
<https://www.kernel.org/pub/linux/utils/util-linux/>.
util-linux 2.38.1 2022-05-11 SWAPON(8)
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