1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
3 .\" %%%LICENSE_START(VERBATIM)
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13 .\" Since the Linux kernel and libraries are constantly changing, this
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18 .\" which is licensed free of charge, as they might when working
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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 .TH SWAPON 2 2014-05-28 "Linux" "Linux Programmer's Manual"
39 swapon, swapoff \- start/stop swapping to file/device
41 .B #include <unistd.h>
43 .B #include <sys/swap.h>
45 .BI "int swapon(const char *" path ", int " swapflags );
47 .BI "int swapoff(const char *" path );
50 sets the swap area to the file or block device specified by
53 stops swapping to the file or block device specified by
58 flag is specified in the
61 argument, the new swap area will have a higher priority than default.
62 The priority is encoded within
67 .I " (prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK"
72 flag is specified in the
75 argument, freed swap pages will be discarded before they are reused,
76 if the swap device supports the discard or trim operation.
77 (This may improve performance on some Solid State Devices,
78 but often it does not.)
81 These functions may be used only by a privileged process (one having the
85 Each swap area has a priority, either high or low.
86 The default priority is low.
87 Within the low-priority areas,
88 newer areas are even lower priority than older areas.
90 All priorities set with
92 are high-priority, higher than default.
93 They may have any nonnegative value chosen by the caller.
94 Higher numbers mean higher priority.
96 Swap pages are allocated from areas in priority order,
97 highest priority first.
98 For areas with different priorities,
99 a higher-priority area is exhausted before using a lower-priority area.
100 If two or more areas have the same priority,
101 and it is the highest priority available,
102 pages are allocated on a round-robin basis between them.
104 As of Linux 1.3.6, the kernel usually follows these rules,
105 but there are exceptions.
107 On success, zero is returned.
108 On error, \-1 is returned, and
110 is set appropriately.
118 is already being used as a swap area.
123 exists, but refers neither to a regular file nor to a block device;
127 The indicated path does not contain a valid swap signature or
128 resides on an in-memory filesystem such as tmpfs.
130 .BR EINVAL " (since Linux 3.4)"
132 An invalid flag value was specified in
138 is not currently a swap area.
141 The system limit on the total number of open files has been reached.
149 The system has insufficient memory to start swapping.
152 The caller does not have the
155 Alternatively, the maximum number of swap files are already in use;
158 These functions are Linux-specific and should not be used in programs
159 intended to be portable.
162 argument was introduced in Linux 1.3.2.
164 The partition or path must be prepared with
167 There is an upper limit on the number of swap files that may be used,
168 defined by the kernel constant
170 Before kernel 2.4.10,
173 since kernel 2.4.10, it has the value 32.
174 Since kernel 2.6.18, the limit is decreased by 2 (thus: 30)
175 if the kernel is built with the
178 (which reserves two swap table entries for the page migration features of
181 .BR migrate_pages (2)).
182 Since kernel 2.6.32, the limit is further decreased by 1
183 if the kernel is built with the
184 .B CONFIG_MEMORY_FAILURE
187 Discard of swap pages was introduced in kernel 2.6.29,
188 then made conditional
191 flag in kernel 2.6.36,
192 .\" To be precise: 2.6.35.5
193 which still discards the
194 entire swap area when
196 is called, even if that flag bit is not set.
202 This page is part of release 3.68 of the Linux
205 A description of the project,
206 information about reporting bugs,
207 and the latest version of this page,
209 \%http://www.kernel.org/doc/man\-pages/.