[MAN] swapon

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SWAPON

Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
Index Return to Main Contents

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NAME

swapon, swapoff - start/stop swapping to file/device  

SYNOPSIS

#include <unistd.h>
#include <sys/swap.h>

int swapon(const char *path, int swapflags);
int swapoff(const char *path);  

DESCRIPTION

swapon() sets the swap area to the file or block device specified by path. swapoff() stops swapping to the file or block device specified by path.

If the SWAP_FLAG_PREFER flag is specified in the swapon() swapflags argument, the new swap area will have a higher priority than default. The priority is encoded within swapflags as:

(prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK

If the SWAP_FLAG_DISCARD flag is specified in the swapon() swapflags argument, freed swap pages will be discarded before they are reused, if the swap device supports the discard or trim operation. (This may improve performance on some Solid State Devices, but often it does not.) See also NOTES.

These functions may be used only by a privileged process (one having the CAP_SYS_ADMIN capability).  

Priority

Each swap area has a priority, either high or low. The default priority is low. Within the low-priority areas, newer areas are even lower priority than older areas.

All priorities set with swapflags are high-priority, higher than default. They may have any nonnegative value chosen by the caller. Higher numbers mean higher priority.

Swap pages are allocated from areas in priority order, highest priority first. For areas with different priorities, a higher-priority area is exhausted before using a lower-priority area. If two or more areas have the same priority, and it is the highest priority available, pages are allocated on a round-robin basis between them.

As of Linux 1.3.6, the kernel usually follows these rules, but there are exceptions.  

RETURN VALUE

On success, zero is returned. On error, -1 is returned, and errno is set appropriately.  

ERRORS

EBUSY

(for swapon()) The specified path is already being used as a swap area.

EINVAL

The file path exists, but refers neither to a regular file nor to a block device;

EINVAL

(swapon()) The indicated path does not contain a valid swap signature or resides on an in-memory filesystem such as tmpfs(5).

EINVAL (since Linux 3.4)

(swapon()) An invalid flag value was specified in flags.

EINVAL

(swapoff()) path is not currently a swap area.

ENFILE

The system-wide limit on the total number of open files has been reached.

ENOENT

The file path does not exist.

ENOMEM

The system has insufficient memory to start swapping.

EPERM

The caller does not have the CAP_SYS_ADMIN capability. Alternatively, the maximum number of swap files are already in use; see NOTES below.

 

CONFORMING TO

These functions are Linux-specific and should not be used in programs intended to be portable. The second swapflags argument was introduced in Linux 1.3.2.  

NOTES

The partition or path must be prepared with mkswap(8).

There is an upper limit on the number of swap files that may be used, defined by the kernel constant MAX_SWAPFILES. Before kernel 2.4.10, MAX_SWAPFILES has the value 8; since kernel 2.4.10, it has the value 32. Since kernel 2.6.18, the limit is decreased by 2 (thus: 30) if the kernel is built with the CONFIG_MIGRATION option (which reserves two swap table entries for the page migration features of mbind(2) and migrate_pages(2)). Since kernel 2.6.32, the limit is further decreased by 1 if the kernel is built with the CONFIG_MEMORY_FAILURE option.

Discard of swap pages was introduced in kernel 2.6.29, then made conditional on the SWAP_FLAG_DISCARD flag in kernel 2.6.36, which still discards the entire swap area when swapon() is called, even if that flag bit is not set.  

SEE ALSO

mkswap(8), swapoff(8), swapon(8)  

COLOPHON

This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

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Index

NAME

SYNOPSIS

DESCRIPTION

Priority

RETURN VALUE

ERRORS

CONFORMING TO

NOTES

SEE ALSO

COLOPHON

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SWAPON

Section: System Administration (8)
Updated: October 2014
Index Return to Main Contents

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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.

-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(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". 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.

 

SEE ALSO

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

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 https://www.kernel.org/pub/linux/utils/util-linux/.

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Index

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

NOTES

ENVIRONMENT

SEE ALSO

FILES

HISTORY

AVAILABILITY

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Time: 04:45:59 GMT, September 16, 2022