//po4a: entry man manual
= swapon(8)
:doctype: manpage
:man manual: System Administration
:man source: util-linux {release-version}
:page-layout: base
:command: swapon

== 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.
See the *FSTAB CONFIGURATION* section for more details.

*-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.

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*--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.

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== FSTAB CONFIGURATION

The command *swapon --all* reads configuration from _/etc/fstab_ (or from a file specified by the *--fstab* command line option). Only fstab entries with the filesystem type (3rd field) set to "swap" are relevant.

The option *--options* accepts values in the same form as can be specified in the fourth field in fstab.

=== The first field (source)

Specify the swap source. If the source is a regular file, it is addressed by an absolute path.

If the swap is a block device, it can be addressed by device path, swap area tags *LABEL=* or *UUID=* (see *mkswap*(8) for more details), or by partition tags like *PARTLABEL=* or *PARTUUID=*.

=== The second field (target)

Unused by *swapon*, the recommended convention is to use "none".

=== The third field (type)

Requires "swap" as the filesystem type.

=== The fourth field (options)

It is formatted as a comma-separated list of options. All unknown options are silently ignored. If options are unnecessary, the recommended convention is to use "defaults". The options specified in _fstab_ extend or overwrite settings specified on the *swapon* command line.

Supported swap options:

*noauto*::
Ignore entry when *swapon --all* is given.

*nofail*::
Do not report errors for this device if it does not exist.

*discard*[**=**_policy_]::
Enable swap discard. The supported settings are *discard*, *discard=once*, or *discard=pages*. For more details, see the **--discard** command line option.

**pri=**_priority_::
Specify the priority of the swap device. For more details, see the **--priority** command line option.

=== The fifth field

Unused by *swapon*, the recommended convention is to keep it empty.

=== The sixth field

Unused by *swapon*, the recommended convention is to keep it empty.

== 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.

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.

== 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)

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