NAME
swapon, swapoff - enable/disable devices and files for paging and swapping
SYNOPSIS
swapon [options] [specialfile...]
swapoff [-va] [specialfile...]
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.
-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.
--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
Print version 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(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.
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.
SEE ALSO
swapoff(2), swapon(2), fstab(5), init(8), fallocate(1), mkswap(8), mount(8), rc(8)
REPORTING BUGS
For bug reports, use the issue tracker at <https://github.com/util-linux/util-linux/issues>;.
AVAILABILITY
The swapon command is part of the util-linux package which can be downloaded from Linux Kernel Archive <https://www.kernel.org/pub/linux/utils/util-linux/>;.