swapon(8) — Linux manual pages

Linux manual pages	Section 8

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.

−h, −−help

Display help text and exit.

−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.

−V, −−version

Display version information 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 syscall failed for another reason

8

non-swapoff 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.

AVAILABILITY

The swapon command is part of the util-linux package and is available from https://www.kernel.org/pub/linux/utils/util-linux/.

  Copyright (c) 1980, 1991 Regents of the University of California.
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.
3. All advertising materials mentioning features or use of this software
   must display the following acknowledgement:
This product includes software developed by the University of
California, Berkeley and its contributors.
4. Neither the name of the University nor the names of its contributors
   may be used to endorse or promote products derived from this software
   without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.

    (#)swapon.8 6.3 (Berkeley) 3/16/91

Name

Synopsis