1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
3 .\" %%%LICENSE_START(VERBATIM)
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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
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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14 .\" manual page may be incorrect or out-of-date.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
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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
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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 2010-11-15 "Linux" "Linux Programmer's Manual"
39 swapon, swapoff \- start/stop swapping to file/device
41 .B #include <unistd.h>
43 .B #include <asm/page.h> /* to find PAGE_SIZE */
45 .B #include <sys/swap.h>
47 .BI "int swapon(const char *" path ", int " swapflags );
49 .BI "int swapoff(const char *" path );
52 sets the swap area to the file or block device specified by
55 stops swapping to the file or block device specified by
60 flag is specified in the
63 argument, the new swap area will have a higher priority than default.
64 The priority is encoded within
69 .I " (prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK"
74 flag is specified in the
77 argument, freed swap pages will be discarded before they are reused,
78 if the swap device supports the discard or trim operation.
79 (This may improve performance on some Solid State Devices,
80 but often it does not.)
83 These functions may be used only by a privileged process (one having the
87 Each swap area has a priority, either high or low.
88 The default priority is low.
89 Within the low-priority areas,
90 newer areas are even lower priority than older areas.
92 All priorities set with
94 are high-priority, higher than default.
95 They may have any nonnegative value chosen by the caller.
96 Higher numbers mean higher priority.
98 Swap pages are allocated from areas in priority order,
99 highest priority first.
100 For areas with different priorities,
101 a higher-priority area is exhausted before using a lower-priority area.
102 If two or more areas have the same priority,
103 and it is the highest priority available,
104 pages are allocated on a round-robin basis between them.
106 As of Linux 1.3.6, the kernel usually follows these rules,
107 but there are exceptions.
109 On success, zero is returned.
110 On error, \-1 is returned, and
112 is set appropriately.
120 is already being used as a swap area.
125 exists, but refers neither to a regular file nor to a block device;
128 the indicated path does not contain a valid swap signature or
129 resides on an in-memory file system like tmpfs; or, for
132 is not currently a swap area.
135 The system limit on the total number of open files has been reached.
143 The system has insufficient memory to start swapping.
146 The caller does not have the
149 Alternatively, the maximum number of swap files are already in use;
152 These functions are Linux-specific and should not be used in programs
153 intended to be portable.
156 argument was introduced in Linux 1.3.2.
158 The partition or path must be prepared with
161 There is an upper limit on the number of swap files that may be used,
162 defined by the kernel constant
164 Before kernel 2.4.10,
167 since kernel 2.4.10, it has the value 32.
168 Since kernel 2.6.18, the limit is decreased by 2 (thus: 30)
169 if the kernel is built with the
172 (which reserves two swap table entries for the page migration features of
175 .BR migrate_pages (2)).
176 Since kernel 2.6.32, the limit is further decreased by 1
177 if the kernel is built with the
178 .B CONFIG_MEMORY_FAILURE
181 Discard of swap pages was introduced in kernel 2.6.29,
182 then made conditional
185 flag in kernel 2.6.36,
186 .\" To be precise: 2.6.35.5
187 which still discards the
188 entire swap area when
190 is called, even if that flag bit is not set.