名前

swapon, swapoff - enable/disable devices and files for paging and swapping

書式

swapon [options] [specialfile...]

swapoff [-va] [specialfile...]

説明

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).

オプション

-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

指定された label を持つパーティションを用いる。 (そのため /proc/partitions へのアクセスが必要である。)

-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

出力可能な項目すべてを出力する。

--noheadings

--show による出力を表示する際に、 ヘッダー行を表示しない。

--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

指定された uuid を持つパーティションを用いる。

-v, --verbose

詳細に表示する。

-h, --help

ヘルプテキストを表示して終了する。

-V, --version

バージョンを表示して、 終了する。

終了ステータス

swapoff has the following exit status values since v2.36:

0

成功。

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.

環境変数

LIBMOUNT_DEBUG=all

libmount デバッグ出力を有効にする。

LIBBLKID_DEBUG=all

libblkid デバッグ出力を有効にする。

ファイル

/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.

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.

履歴

swapon コマンドは 4.0BSD から登場した。

関連項目

swapoff(2), swapon(2), fstab(5), init(8), fallocate(1), mkswap(8), mount(8), rc(8)

バグ報告

バグ報告は、 以下に示す issue トラッカーを利用すること。 <https://github.com/util-linux/util-linux/issues>.

入手方法

swapon コマンドは util-linux パッケージの一部であり、 以下からダウンロードできる。 Linux Kernel Archive <https://www.kernel.org/pub/linux/utils/util-linux/>.