swapon, swapoff — start/stop swapping to file/device
#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); |
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).
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.
On success, zero is returned. On error, −1 is
returned, and errno is set
appropriately.
(for swapon()) The
specified path
is already being used as a swap area.
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.
The system limit on the total number of open files has been reached.
The file path does not exist.
The system has insufficient memory to start swapping.
The caller does not have the CAP_SYS_ADMIN capability.
Alternatively, the maximum number of swap files are
already in use; see NOTES below.
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.
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.
This page is part of release 3.25 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 professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. 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. |