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

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 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

Anhang

Siehe auch

---

---

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