man2/swapon.2

   1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
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  24 .\"
  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.
  36 .\"
  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
  41 .\"
  42 .TH SWAPON 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  43 .SH NAME
  44 swapon, swapoff \- start/stop swapping to file/device
  45 .SH SYNOPSIS
  46 .B #include <unistd.h>
  47 .br
  48 .B #include <sys/swap.h>
  49 .PP
  50 .BI "int swapon(const char *" path ", int " swapflags );
  51 .br
  52 .BI "int swapoff(const char *" path );
  53 .SH DESCRIPTION
  54 .BR swapon ()
  55 sets the swap area to the file or block device specified by
  56 .IR path .
  57 .BR swapoff ()
  58 stops swapping to the file or block device specified by
  59 .IR path .
  60 .PP
  61 If the
  62 .B SWAP_FLAG_PREFER
  63 flag is specified in the
  64 .BR swapon ()
  65 .I swapflags
  66 argument, the new swap area will have a higher priority than default.
  67 The priority is encoded within
  68 .I swapflags
  69 as:
  70 .PP
  71 .in +4n
  72 .EX
  73 .I "(prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK"
  74 .EE
  75 .in
  76 .PP
  77 If the
  78 .B SWAP_FLAG_DISCARD
  79 flag is specified in the
  80 .BR swapon ()
  81 .I swapflags
  82 argument, freed swap pages will be discarded before they are reused,
  83 if the swap device supports the discard or trim operation.
  84 (This may improve performance on some Solid State Devices,
  85 but often it does not.)
  86 See also NOTES.
  87 .PP
  88 These functions may be used only by a privileged process (one having the
  89 .B CAP_SYS_ADMIN
  90 capability).
  91 .SS Priority
  92 Each swap area has a priority, either high or low.
  93 The default priority is low.
  94 Within the low-priority areas,
  95 newer areas are even lower priority than older areas.
  96 .PP
  97 All priorities set with
  98 .I swapflags
  99 are high-priority, higher than default.
 100 They may have any nonnegative value chosen by the caller.
 101 Higher numbers mean higher priority.
 102 .PP
 103 Swap pages are allocated from areas in priority order,
 104 highest priority first.
 105 For areas with different priorities,
 106 a higher-priority area is exhausted before using a lower-priority area.
 107 If two or more areas have the same priority,
 108 and it is the highest priority available,
 109 pages are allocated on a round-robin basis between them.
 110 .PP
 111 As of Linux 1.3.6, the kernel usually follows these rules,
 112 but there are exceptions.
 113 .SH RETURN VALUE
 114 On success, zero is returned.
 115 On error, \-1 is returned, and
 116 .I errno
 117 is set appropriately.
 118 .SH ERRORS
 119 .TP
 120 .B EBUSY
 121 (for
 122 .BR swapon ())
 123 The specified
 124 .I path
 125 is already being used as a swap area.
 126 .TP
 127 .B EINVAL
 128 The file
 129 .I path
 130 exists, but refers neither to a regular file nor to a block device;
 131 .TP
 132 .B EINVAL
 133 .RB ( swapon ())
 134 The indicated path does not contain a valid swap signature or
 135 resides on an in-memory filesystem such as
 136 .BR tmpfs (5).
 137 .TP
 138 .BR EINVAL " (since Linux 3.4)"
 139 .RB ( swapon ())
 140 An invalid flag value was specified in
 141 .IR flags .
 142 .TP
 143 .B EINVAL
 144 .RB ( swapoff ())
 145 .I path
 146 is not currently a swap area.
 147 .TP
 148 .B ENFILE
 149 The system-wide limit on the total number of open files has been reached.
 150 .TP
 151 .B ENOENT
 152 The file
 153 .I path
 154 does not exist.
 155 .TP
 156 .B ENOMEM
 157 The system has insufficient memory to start swapping.
 158 .TP
 159 .B EPERM
 160 The caller does not have the
 161 .B CAP_SYS_ADMIN
 162 capability.
 163 Alternatively, the maximum number of swap files are already in use;
 164 see NOTES below.
 165 .SH CONFORMING TO
 166 These functions are Linux-specific and should not be used in programs
 167 intended to be portable.
 168 The second
 169 .I swapflags
 170 argument was introduced in Linux 1.3.2.
 171 .SH NOTES
 172 The partition or path must be prepared with
 173 .BR mkswap (8).
 174 .PP
 175 There is an upper limit on the number of swap files that may be used,
 176 defined by the kernel constant
 177 .BR MAX_SWAPFILES .
 178 Before kernel 2.4.10,
 179 .B MAX_SWAPFILES
 180 has the value 8;
 181 since kernel 2.4.10, it has the value 32.
 182 Since kernel 2.6.18, the limit is decreased by 2 (thus: 30)
 183 if the kernel is built with the
 184 .B CONFIG_MIGRATION
 185 option
 186 (which reserves two swap table entries for the page migration features of
 187 .BR mbind (2)
 188 and
 189 .BR migrate_pages (2)).
 190 Since kernel 2.6.32, the limit is further decreased by 1
 191 if the kernel is built with the
 192 .B CONFIG_MEMORY_FAILURE
 193 option.
 194 .PP
 195 Discard of swap pages was introduced in kernel 2.6.29,
 196 then made conditional
 197 on the
 198 .B SWAP_FLAG_DISCARD
 199 flag in kernel 2.6.36,
 200 .\" To be precise: 2.6.35.5
 201 which still discards the
 202 entire swap area when
 203 .BR swapon ()
 204 is called, even if that flag bit is not set.
 205 .SH SEE ALSO
 206 .BR mkswap (8),
 207 .BR swapoff (8),
 208 .BR swapon (8)