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.

swapon() takes a swapflags argument. If swapflags has the SWAP_FLAG_PREFER bit turned on, 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

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 non-negative 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 filesystem 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 2.6.10, MAX_SWAPFILES has the value 8; since kernel 2.6.10, 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)).

SEE ALSO

mkswap(8), swapoff(8), swapon(8)

COLOPHON

This page is part of release 2.79 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/.


Copyright (c) 1992 Drew Eckhardt (drewcs.colorado.edu), March 28, 1992

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
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Formatted or processed versions of this manual, if unaccompanied by
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Modified by Michael Haardt <michaelmoria.de>
Modified 1993-07-24 by Rik Faith <faithcs.unc.edu>
Modified 1995-07-22 by Michael Chastain <mecduracef.shout.net>
Modified 1995-07-23 by aeb
Modified 1996-10-22 by Eric S. Raymond <esrthyrsus.com>
Modified 1998-09-08 by aeb
Modified 2004-06-17 by Michael Kerrisk <mtk.manpagesgmail.com>
Modified 2004-10-10 by aeb
2004-12-14 mtk, Anand Kumria: added new errors
2007-06-22 Ivana Varekova <varekovaredhat.com>, mtk
    Update text describing limit on number of swap files.