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 2015-12-28 "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 .sp
  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 .br
  71 .sp
  72 .I "    (prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK"
  73 .br
  74 .PP
  75 If the
  76 .B SWAP_FLAG_DISCARD
  77 flag is specified in the
  78 .BR swapon ()
  79 .I swapflags
  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.)
  84 See also NOTES.
  85 .PP
  86 These functions may be used only by a privileged process (one having the
  87 .B CAP_SYS_ADMIN
  88 capability).
  89 .SS Priority
  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.
  94 .PP
  95 All priorities set with
  96 .I swapflags
  97 are high-priority, higher than default.
  98 They may have any nonnegative value chosen by the caller.
  99 Higher numbers mean higher priority.
 100 .PP
 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.
 108 .PP
 109 As of Linux 1.3.6, the kernel usually follows these rules,
 110 but there are exceptions.
 111 .SH RETURN VALUE
 112 On success, zero is returned.
 113 On error, \-1 is returned, and
 114 .I errno
 115 is set appropriately.
 116 .SH ERRORS
 117 .TP
 118 .B EBUSY
 119 (for
 120 .BR swapon ())
 121 The specified
 122 .I path
 123 is already being used as a swap area.
 124 .TP
 125 .B EINVAL
 126 The file
 127 .I path
 128 exists, but refers neither to a regular file nor to a block device;
 129 .TP
 130 .B EINVAL
 131 .RB ( swapon ())
 132 The indicated path does not contain a valid swap signature or
 133 resides on an in-memory filesystem such as
 134 .BR tmpfs (5).
 135 .TP
 136 .BR EINVAL " (since Linux 3.4)"
 137 .RB ( swapon ())
 138 An invalid flag value was specified in
 139 .IR flags .
 140 .TP
 141 .B EINVAL
 142 .RB ( swapoff ())
 143 .I path
 144 is not currently a swap area.
 145 .TP
 146 .B ENFILE
 147 The system-wide limit on the total number of open files has been reached.
 148 .TP
 149 .B ENOENT
 150 The file
 151 .I path
 152 does not exist.
 153 .TP
 154 .B ENOMEM
 155 The system has insufficient memory to start swapping.
 156 .TP
 157 .B EPERM
 158 The caller does not have the
 159 .B CAP_SYS_ADMIN
 160 capability.
 161 Alternatively, the maximum number of swap files are already in use;
 162 see NOTES below.
 163 .SH CONFORMING TO
 164 These functions are Linux-specific and should not be used in programs
 165 intended to be portable.
 166 The second
 167 .I swapflags
 168 argument was introduced in Linux 1.3.2.
 169 .SH NOTES
 170 The partition or path must be prepared with
 171 .BR mkswap (8).
 172
 173 There is an upper limit on the number of swap files that may be used,
 174 defined by the kernel constant
 175 .BR MAX_SWAPFILES .
 176 Before kernel 2.4.10,
 177 .B MAX_SWAPFILES
 178 has the value 8;
 179 since kernel 2.4.10, it has the value 32.
 180 Since kernel 2.6.18, the limit is decreased by 2 (thus: 30)
 181 if the kernel is built with the
 182 .B CONFIG_MIGRATION
 183 option
 184 (which reserves two swap table entries for the page migration features of
 185 .BR mbind (2)
 186 and
 187 .BR migrate_pages (2)).
 188 Since kernel 2.6.32, the limit is further decreased by 1
 189 if the kernel is built with the
 190 .B CONFIG_MEMORY_FAILURE
 191 option.
 192
 193 Discard of swap pages was introduced in kernel 2.6.29,
 194 then made conditional
 195 on the
 196 .B SWAP_FLAG_DISCARD
 197 flag in kernel 2.6.36,
 198 .\" To be precise: 2.6.35.5
 199 which still discards the
 200 entire swap area when
 201 .BR swapon ()
 202 is called, even if that flag bit is not set.
 203 .SH SEE ALSO
 204 .BR mkswap (8),
 205 .BR swapoff (8),
 206 .BR swapon (8)