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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | EXIT STATUS | ENVIRONMENT | FILES | NOTES | HISTORY | SEE ALSO | REPORTING BUGS | AVAILABILITY |
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SWAPON(8) System Administration SWAPON(8)
swapon, swapoff - enable/disable devices and files for paging and
swapping
swapon [options] [specialfile...]
swapoff [-va] [specialfile...]
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.
-T, --fstab path
Specifies an alternative fstab file for compatibility with
mount(8). If path is a directory, then the files in the
directory are sorted by strverscmp(3); files that start with
"." or without an .fstab extension are ignored. The option can
be specified more than once. This option is mostly designed
for initramfs or chroot scripts where additional configuration
is specified beyond standard system configuration.
-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 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. When no priority is defined, Linux kernel defaults
to negative numbers.
-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
Display version and exit.
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.
LIBMOUNT_DEBUG=all
enables libmount debug output.
LIBBLKID_DEBUG=all
enables libblkid debug output.
/dev/sd??
standard paging devices
/etc/fstab
ascii filesystem description table
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.
Since version 2.41, the command mkswap --file can create a new
swap file with the nocow attribute.
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.
The swapon command appeared in 4.0BSD.
swapoff(2), swapon(2), fstab(5), init(8), fallocate(1), mkswap(8),
mount(8), rc(8)
For bug reports, use the issue tracker
<https://github.com/util-linux/util-linux/issues>.
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/>. This page is
part of the util-linux (a random collection of Linux utilities)
project. Information about the project can be found at
⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have a
bug report for this manual page, send it to
[email protected]. This page was obtained from the
project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
2025-08-11. (At that time, the date of the most recent commit that
was found in the repository was 2025-08-05.) If you discover any
rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to [email protected]
util-linux 2.42-start-521-ec46 2025-08-09 SWAPON(8)
Pages that refer to this page: swapon(2), fstab(5), org.freedesktop.systemd1(5), proc_swaps(5), systemd.swap(5), mkswap(8), mount(8), swaplabel(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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