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 avaliable 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(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/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". Not available before Linux 2.1.25. 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.

−−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. This can be seen in the system log as

swapon: swapfile has 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 preallocated files (e.g. fallocate(1)) on filesystems like XFS or ext4, and on copy-on-write filesystems like btrfs.

It is recommended to use dd(1) and /dev/zero to avoid holes on XFS and ext4.

swapon may not work correctly when using a swap file with some versions of btrfs. This is due to btrfs being a copy-on-write filesystem: the file location may not be static and corruption can result. Btrfs actively disallows the use of swap files on its filesystems by refusing to map the file.

One possible workaround is to map the swap file to a loopback device. This will allow the filesystem to determine the mapping properly but may come with a performance impact.

Swap over NFS may not work.

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.

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

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.kernel.org/pub/linux/utils/util-linux/.

  Copyright (c) 1980, 1991 Regents of the University of California.
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    (#)swapon.8 6.3 (Berkeley) 3/16/91

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