sys-utils/swapon.8

   1 .\" Copyright (c) 1980, 1991 Regents of the University of California.
   2 .\" All rights reserved.
   3 .\"
   4 .\" Redistribution and use in source and binary forms, with or without
   5 .\" modification, are permitted provided that the following conditions
   6 .\" are met:
   7 .\" 1. Redistributions of source code must retain the above copyright
   8 .\"    notice, this list of conditions and the following disclaimer.
   9 .\" 2. Redistributions in binary form must reproduce the above copyright
  10 .\"    notice, this list of conditions and the following disclaimer in the
  11 .\"    documentation and/or other materials provided with the distribution.
  12 .\" 3. All advertising materials mentioning features or use of this software
  13 .\"    must display the following acknowledgement:
  14 .\"     This product includes software developed by the University of
  15 .\"     California, Berkeley and its contributors.
  16 .\" 4. Neither the name of the University nor the names of its contributors
  17 .\"    may be used to endorse or promote products derived from this software
  18 .\"    without specific prior written permission.
  19 .\"
  20 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  21 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  22 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  23 .\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  24 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  25 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  26 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  27 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  28 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  29 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  30 .\" SUCH DAMAGE.
  31 .\"
  32 .\"     @(#)swapon.8    6.3 (Berkeley) 3/16/91
  33 .\"
  34 .TH SWAPON 8 "October 2014" "util-linux" "System Administration"
  35 .SH NAME
  36 swapon, swapoff \- enable/disable devices and files for paging and swapping
  37 .SH SYNOPSIS
  38 .BR swapon
  39 [options]
  40 .RI [ specialfile ...]
  41 .br
  42 .B swapoff
  43 .RB [ \-va ]
  44 .RI [ specialfile ...]
  45 .SH DESCRIPTION
  46 .B swapon
  47 is used to specify devices on which paging and swapping are to take place.
  48
  49 The device or file used is given by the
  50 .I specialfile
  51 parameter.  It may be of the form
  52 .BI \-L " label"
  53 or
  54 .BI \-U " uuid"
  55 to indicate a device by label or uuid.
  56
  57 Calls to
  58 .B swapon
  59 normally occur in the system boot scripts making all swap devices available, so
  60 that the paging and swapping activity is interleaved across several devices and
  61 files.
  62
  63 .B swapoff
  64 disables swapping on the specified devices and files.
  65 When the
  66 .B \-a
  67 flag is given, swapping is disabled on all known swap devices and files
  68 (as found in
  69 .I /proc/swaps
  70 or
  71 .IR /etc/fstab ).
  72
  73 .SH OPTIONS
  74 .TP
  75 .BR \-a , " \-\-all"
  76 All devices marked as ``swap'' in
  77 .I /etc/fstab
  78 are made available, except for those with the ``noauto'' option.
  79 Devices that are already being used as swap are silently skipped.
  80 .TP
  81 .BR \-d , " \-\-discard" [ =\fIpolicy\fR]
  82 Enable swap discards, if the swap backing device supports the discard or
  83 trim operation.  This may improve performance on some Solid State Devices,
  84 but often it does not.  The option allows one to select between two
  85 available swap discard policies:
  86 .BI \-\-discard=once
  87 to perform a single-time discard operation for the whole swap area at swapon;
  88 or
  89 .BI \-\-discard=pages
  90 to asynchronously discard freed swap pages before they are available for reuse.
  91 If no policy is selected, the default behavior is to enable both discard types.
  92 The
  93 .I /etc/fstab
  94 mount options
  95 .BR discard ,
  96 .BR discard=once ,
  97 or
  98 .B discard=pages
  99 may also be used to enable discard flags.
 100 .TP
 101 .BR \-e , " \-\-ifexists"
 102 Silently skip devices that do not exist.
 103 The
 104 .I /etc/fstab
 105 mount option
 106 .B nofail
 107 may also be used to skip non-existing device.
 108
 109 .TP
 110 .BR \-f , " \-\-fixpgsz"
 111 Reinitialize (exec mkswap) the swap space if its page size does not
 112 match that of the current running kernel.
 113 .BR mkswap (2)
 114 initializes the whole device and does not check for bad blocks.
 115 .TP
 116 .BR \-h , " \-\-help"
 117 Display help text and exit.
 118 .TP
 119 .BI \-L " label"
 120 Use the partition that has the specified
 121 .IR label .
 122 (For this, access to
 123 .I /proc/partitions
 124 is needed.)
 125 .TP
 126 .BR \-o , " \-\-options " \fIopts\fP
 127 Specify swap options by an fstab-compatible comma-separated string.
 128 For example:
 129 .RS
 130 .RS
 131 .sp
 132 .B "swapon -o pri=1,discard=pages,nofail /dev/sda2"
 133 .sp
 134 .RE
 135 The \fIopts\fP string is evaluated last and overrides all other
 136 command line options.
 137 .RE
 138 .TP
 139 .BR \-p , " \-\-priority " \fIpriority\fP
 140 Specify the priority of the swap device.
 141 .I priority
 142 is a value between \-1 and 32767.  Higher numbers indicate
 143 higher priority.  See
 144 .BR swapon (2)
 145 for a full description of swap priorities.  Add
 146 .BI pri= value
 147 to the option field of
 148 .I /etc/fstab
 149 for use with
 150 .BR "swapon -a" .
 151 When no priority is defined, it defaults to \-1.
 152 .TP
 153 .BR \-s , " \-\-summary"
 154 Display swap usage summary by device.  Equivalent to "cat /proc/swaps".
 155 This output format is DEPRECATED in favour
 156 of \fB\-\-show\fR that provides better control on output data.
 157 .TP
 158 .BR \-\-show [ =\fIcolumn\fR ...]
 159 Display a definable table of swap areas.  See the
 160 .B \-\-help
 161 output for a list of available columns.
 162 .TP
 163 .B \-\-noheadings
 164 Do not print headings when displaying
 165 .B \-\-show
 166 output.
 167 .TP
 168 .B \-\-raw
 169 Display
 170 .B \-\-show
 171 output without aligning table columns.
 172 .TP
 173 .B \-\-bytes
 174 Display swap size in bytes in
 175 .B \-\-show
 176 output instead of in user-friendly units.
 177 .TP
 178 .BI \-U  " uuid"
 179 Use the partition that has the specified
 180 .IR uuid .
 181 .TP
 182 .BR \-v , " \-\-verbose"
 183 Be verbose.
 184 .TP
 185 .BR \-V , " \-\-version"
 186 Display version information and exit.
 187 .SH NOTES
 188 You should not use \fBswapon\fR on a file with holes.
 189 This can be seen in the system log as
 190 .RS
 191 .sp
 192 .B "swapon: swapfile has holes."
 193 .sp
 194 .RE
 195 The swap file implementation in the kernel expects to be able to write to the
 196 file directly, without the assistance of the filesystem.  This is a problem on
 197 preallocated files (e.g.
 198 .BR fallocate (1))
 199 on filesystems like \fBXFS\fR or \fBext4\fR, and on copy-on-write
 200 filesystems like \fBbtrfs\fR.
 201 .PP
 202 It is recommended to use
 203 .BR dd (1)
 204 and
 205 .I /dev/zero
 206 to avoid holes on XFS and ext4.
 207 .PP
 208 .B swapon
 209 may not work correctly when using a swap file with some versions of
 210 \fBbtrfs\fR.  This is due to btrfs being a copy-on-write filesystem: the
 211 file location may not be static and corruption can result.  Btrfs actively
 212 disallows the use of swap files on its filesystems by refusing to map the file.
 213 .PP
 214 One possible workaround is to map the swap
 215 file to a loopback device.  This will allow the filesystem to determine the
 216 mapping properly but may come with a performance impact.
 217 .PP
 218 Swap over \fBNFS\fR may not work.
 219 .PP
 220 .B swapon
 221 automatically detects and rewrites a swap space signature with old software
 222 suspend data (e.g S1SUSPEND, S2SUSPEND, ...). The problem is that if we don't
 223 do it, then we get data corruption the next time an attempt at unsuspending is
 224 made.
 225
 226 .SH ENVIRONMENT
 227 .IP LIBMOUNT_DEBUG=all
 228 enables libmount debug output.
 229 .IP LIBBLKID_DEBUG=all
 230 enables libblkid debug output.
 231
 232 .SH SEE ALSO
 233 .BR swapoff (2),
 234 .BR swapon (2),
 235 .BR fstab (5),
 236 .BR init (8),
 237 .BR mkswap (8),
 238 .BR mount (8),
 239 .BR rc (8)
 240 .SH FILES
 241 .br
 242 .I /dev/sd??
 243 standard paging devices
 244 .br
 245 .I /etc/fstab
 246 ascii filesystem description table
 247 .SH HISTORY
 248 The
 249 .B swapon
 250 command appeared in 4.0BSD.
 251 .SH AVAILABILITY
 252 The swapon command is part of the util-linux package and is available from
 253 https://www.kernel.org/pub/linux/utils/util-linux/.