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swapon

, swapoff start/stop swapping to file/device

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1. swapon.2.man

Manpage of SWAPON

SWAPON

Section: Linux Programmer's Manual (2)
Updated: 2010-11-15
Index Return to Main Contents

NAME

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

SYNOPSIS

#include <unistd.h>
#include <asm/page.h> /* to find PAGE_SIZE */
#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 only be used 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; or, for swapon(), the indicated path does not contain a valid swap signature or resides on an in-memory file system like tmpfs; or, for swapoff(), path is not currently a swap area.
ENFILE: The system 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.

COLOPHON

This page is part of release 3.32 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Index

NAME

SYNOPSIS

DESCRIPTION

Priority

This document was created by man2html using the manual pages.
Time: 17:33:01 GMT, May 11, 2012

2. swapon.8.man

Manpage of SWAPON

SWAPON

Section: Linux Programmer's Manual (8)
Updated: 25 September 1995
Index Return to Main Contents

NAME

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

SYNOPSIS

Get info:
swapon -s [-h] [-V]

Enable/disable:
swapon [-d] [-f] [-p priority] [-v] specialfile...
swapoff [-v] specialfile...

Enable/disable all:
swapon -a [-e] [-f] [-v]
swapoff -a [-v]

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).

-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: Discard freed swap pages 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. The /etc/fstab mount option discard may be also used to enable discard flag.
-e, --ifexists: Silently skip devices that do not exist. The /etc/fstab mount option nofail may be also used to skip non-existing device.
-f, --fixpgsz: Reinitialize (exec /sbin/mkswap) the swap space if its page size does not match that of the the current running kernel. mkswap(2) initializes the whole device and does not check for bad blocks.
-h, --help: Provide help.
-L label: Use the partition that has the specified label. (For this, access to /proc/partitions is needed.)
-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.
-s, --summary: Display swap usage summary by device. Equivalent to "cat /proc/swaps". Not available before Linux 2.1.25.
-U uuid: Use the partition that has the specified uuid.
-v, --verbose: Be verbose.
-V, --version: Display version.

NOTES

You should not use swapon on a file with holes. Swap over NFS may not work.

swapon automatically detects and rewrites 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.

swapon may not work correctly when using a swap file with some versions of btrfs. This is due to the swap file implementation in the kernel expecting to be able to write to the file directly, without the assistance of the file system. Since btrfs is a copy-on-write file system, the file location may not be static and corruption can result. Btrfs actively disallows the use of files on its file systems by refusing to map the file. This can be seen in the system log as "swapon: swapfile has holes." One possible workaround is to map the file to a loopback device. This will allow the file system to determine the mapping properly but may come with a performance impact.

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 ftp://ftp.kernel.org/pub/linux/utils/util-linux/.

This document was created by man2html using the manual pages.
Time: 17:33:01 GMT, May 11, 2012