Swapon

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

Beschreibung

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)

Installation

swapon und swapoff sind Teil von util-linux

Aufruf

swapon [options] [specialfile...]

swapoff [-va] [specialfile...]

Optionen

Unix	GNU	Parameter	Beschreibung
-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
	--annotate	[=when]	Adds an annotation to each column header name. Such an annotation can be shown as a tooltip by terminals that support this feature. The optional when argument can be always, never, or auto. If the argument is omitted, it defaults to auto, which means that annotations will only be used when the output goes to a terminal
	--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

Umgebungsvariablen

Variable	Beschreibung
LIBMOUNT_DEBUG=all	enables libmount debug output
LIBBLKID_DEBUG=all	enables libblkid debug output

Exit-Status

Wert	Beschreibung
0	Erfolg
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)

Anwendung

Problembehebung

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

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

error message

sudo swapon /swap-file-00

swapon: /swap-file-00: skipping - it appears to have holes

solution

you need to zero-fill the file you are using as swap

1. Remove the existing file with the holes

sudo rm /swap-file-00

2. create the new swap file using the following command:

sudo dd if=/dev/zero of=/swap-file-00 bs=1M count=5120 status=progress

5322571776 bytes (5.3 GB, 5.0 GiB) copied, 6 s, 887 MB/s
5120+0 records in
5120+0 records out
5368709120 bytes (5.4 GB, 5.0 GiB) copied, 6.08009 s, 883 MB/s

3. check the file type being displayed now

• it’s just a data file

sudo file /swap-file-00

/swap-file-00: data

if your system says something like the following, it just means that the file command is not installed on your system

• you may either ignore the file commands in the further steps since they are not required in getting a swap file operational on your system
• they are being used to verify the file contents based on their magic numbers
• or you may chose to install the file command using the package manager on your gnu/linux, say apt -y install file on a debian/ubuntu:

file: command not found

change the permissions on it:

sudo chmod 600 /swap-file-00

4. make the new file as swap file

sudo mkswap /swap-file-00

Setting up swapspace version 1, size = 5 GiB (5368705024 bytes)
no label, UUID=738f253b-aed0-46ac-bc50-b4d1fe2d1702

5. check the file type being displayed now

• it’s a swap file

sudo file /swap-file-00

/swap-file-00: Linux/i386 swap file (new style), version 1 (4K pages), size 1310719 pages, no label, UUID=738f253b-aed0-46ac-bc50-b4d1fe2d1702

6. turn on the swap, as usual

sudo swapon /swap-file-00

7. check if it is being used as a swap file

sudo swapon --show

NAME TYPE SIZE USED PRIO
/swap-file-00 file 5G 0B -2

Konfiguration

Dateien

Datei	Beschreibung
/dev/sd??	standard paging devices
/etc/fstab	ascii filesystem description table

fstab-Konfiguration

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

Option	Beschreibung
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

Anhang

Siehe auch

---

---

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