1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
3 .\" %%%LICENSE_START(VERBATIM)
4 .\" Permission is granted to make and distribute verbatim copies of this
5 .\" manual provided the copyright notice and this permission notice are
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8 .\" Permission is granted to copy and distribute modified versions of this
9 .\" manual under the conditions for verbatim copying, provided that the
10 .\" entire resulting derived work is distributed under the terms of a
11 .\" permission notice identical to this one.
13 .\" Since the Linux kernel and libraries are constantly changing, this
14 .\" manual page may be incorrect or out-of-date. The author(s) assume no
15 .\" responsibility for errors or omissions, or for damages resulting from
16 .\" the use of the information contained herein. The author(s) may not
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18 .\" which is licensed free of charge, as they might when working
21 .\" Formatted or processed versions of this manual, if unaccompanied by
22 .\" the source, must acknowledge the copyright and authors of this work.
25 .\" Modified by Michael Haardt <michael@moria.de>
26 .\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
27 .\" Modified 1995-07-22 by Michael Chastain <mec@duracef.shout.net>
28 .\" Modified 1995-07-23 by aeb
29 .\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
30 .\" Modified 1998-09-08 by aeb
31 .\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
32 .\" Modified 2004-10-10 by aeb
33 .\" 2004-12-14 mtk, Anand Kumria: added new errors
34 .\" 2007-06-22 Ivana Varekova <varekova@redhat.com>, mtk
35 .\" Update text describing limit on number of swap files.
37 .\" FIXME Linux 3.11 added SWAP_FLAG_DISCARD_ONCE and SWAP_FLAG_DISCARD_PAGES
38 .\" commit dcf6b7ddd7df8965727746f89c59229b23180e5a
39 .\" Author: Rafael Aquini <aquini@redhat.com>
40 .\" Date: Wed Jul 3 15:02:46 2013 -0700
42 .TH SWAPON 2 2014-05-28 "Linux" "Linux Programmer's Manual"
44 swapon, swapoff \- start/stop swapping to file/device
46 .B #include <unistd.h>
48 .B #include <sys/swap.h>
50 .BI "int swapon(const char *" path ", int " swapflags );
52 .BI "int swapoff(const char *" path );
55 sets the swap area to the file or block device specified by
58 stops swapping to the file or block device specified by
63 flag is specified in the
66 argument, the new swap area will have a higher priority than default.
67 The priority is encoded within
72 .I " (prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK"
77 flag is specified in the
80 argument, freed swap pages will be discarded before they are reused,
81 if the swap device supports the discard or trim operation.
82 (This may improve performance on some Solid State Devices,
83 but often it does not.)
86 These functions may be used only by a privileged process (one having the
90 Each swap area has a priority, either high or low.
91 The default priority is low.
92 Within the low-priority areas,
93 newer areas are even lower priority than older areas.
95 All priorities set with
97 are high-priority, higher than default.
98 They may have any nonnegative value chosen by the caller.
99 Higher numbers mean higher priority.
101 Swap pages are allocated from areas in priority order,
102 highest priority first.
103 For areas with different priorities,
104 a higher-priority area is exhausted before using a lower-priority area.
105 If two or more areas have the same priority,
106 and it is the highest priority available,
107 pages are allocated on a round-robin basis between them.
109 As of Linux 1.3.6, the kernel usually follows these rules,
110 but there are exceptions.
112 On success, zero is returned.
113 On error, \-1 is returned, and
115 is set appropriately.
123 is already being used as a swap area.
128 exists, but refers neither to a regular file nor to a block device;
132 The indicated path does not contain a valid swap signature or
133 resides on an in-memory filesystem such as tmpfs.
135 .BR EINVAL " (since Linux 3.4)"
137 An invalid flag value was specified in
143 is not currently a swap area.
146 The system limit on the total number of open files has been reached.
154 The system has insufficient memory to start swapping.
157 The caller does not have the
160 Alternatively, the maximum number of swap files are already in use;
163 These functions are Linux-specific and should not be used in programs
164 intended to be portable.
167 argument was introduced in Linux 1.3.2.
169 The partition or path must be prepared with
172 There is an upper limit on the number of swap files that may be used,
173 defined by the kernel constant
175 Before kernel 2.4.10,
178 since kernel 2.4.10, it has the value 32.
179 Since kernel 2.6.18, the limit is decreased by 2 (thus: 30)
180 if the kernel is built with the
183 (which reserves two swap table entries for the page migration features of
186 .BR migrate_pages (2)).
187 Since kernel 2.6.32, the limit is further decreased by 1
188 if the kernel is built with the
189 .B CONFIG_MEMORY_FAILURE
192 Discard of swap pages was introduced in kernel 2.6.29,
193 then made conditional
196 flag in kernel 2.6.36,
197 .\" To be precise: 2.6.35.5
198 which still discards the
199 entire swap area when
201 is called, even if that flag bit is not set.
207 This page is part of release 3.79 of the Linux
210 A description of the project,
211 information about reporting bugs,
212 and the latest version of this page,
214 \%http://www.kernel.org/doc/man\-pages/.