sys-utils/swapon.8

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  32 .\"     @(#)swapon.8    6.3 (Berkeley) 3/16/91
  33 .\"
  34 .\" Sun Dec 27 12:31:30 1992: Modified by faith@cs.unc.edu
  35 .\" Sat Mar  6 20:46:02 1993: Modified by faith@cs.unc.edu
  36 .\" Sat Oct  9 09:35:30 1993: Converted to man format by faith@cs.unc.edu
  37 .\" Sat Nov 27 20:22:42 1993: Updated authorship information, faith@cs.unc.edu
  38 .\" Mon Sep 25 14:12:38 1995: Added -v and -p information
  39 .\" Tue Apr 30 03:32:07 1996: Added some text from A. Koppenhoefer
  40 .\"
  41 .TH SWAPON 8 "September 1995" "util-linux" "System Administration"
  42 .SH NAME
  43 swapon, swapoff \- enable/disable devices and files for paging and swapping
  44 .SH SYNOPSIS
  45 .B swapon
  46 [ options ]
  47 .RI [ specialfile... ]
  48 .br
  49 .B swapoff
  50 .RB [ \-va ]
  51 .RI [ specialfile... ]
  52 .SH DESCRIPTION
  53 .B swapon
  54 is used to specify devices on which paging and swapping are to take place.
  55
  56 The device or file used is given by the
  57 .I specialfile
  58 parameter. It may be of the form
  59 .BI \-L " label"
  60 or
  61 .BI \-U " uuid"
  62 to indicate a device by label or uuid.
  63
  64 Calls to
  65 .B swapon
  66 normally occur in the system boot scripts making all swap devices available, so
  67 that the paging and swapping activity is interleaved across several devices and
  68 files.
  69
  70 .B swapoff
  71 disables swapping on the specified devices and files.
  72 When the
  73 .B \-a
  74 flag is given, swapping is disabled on all known swap devices and files
  75 (as found in
  76 .I /proc/swaps
  77 or
  78 .IR /etc/fstab ).
  79
  80 .TP
  81 .B "\-a, \-\-all"
  82 All devices marked as ``swap'' in
  83 .I /etc/fstab
  84 are made available, except for those with the ``noauto'' option.
  85 Devices that are already being used as swap are silently skipped.
  86 .TP
  87 .B "\-d, \-\-discard\fR [=\fIpolicy\fR]"
  88 Enable swap discards, if the swap backing device supports the discard or
  89 trim operation. This may improve performance on some Solid State Devices,
  90 but often it does not. The option allows one to select between two
  91 available swap discard policies:
  92 .BI \-\-discard=once
  93 to perform a single-time discard operation for the whole swap area at swapon;
  94 or
  95 .BI \-\-discard=pages
  96 to discard freed swap pages before they are reused, while swapping.
  97 If no policy is selected, the default behavior is to enable both discard types.
  98 The
  99 .I /etc/fstab
 100 mount options
 101 .BI discard,
 102 .BI discard=once,
 103 or
 104 .BI discard=pages
 105 may be also used to enable discard flags.
 106 .TP
 107 .B "\-e, \-\-ifexists"
 108 Silently skip devices that do not exist.
 109 The
 110 .I /etc/fstab
 111 mount option
 112 .BI nofail
 113 may be also used to skip non-existing device.
 114
 115 .TP
 116 .B "\-f, \-\-fixpgsz"
 117 Reinitialize (exec /sbin/mkswap) the swap space if its page size does not
 118 match that of the current running kernel.
 119 .BR mkswap (2)
 120 initializes the whole device and does not check for bad blocks.
 121 .TP
 122 .B \-h, \-\-help
 123 Display help text and exit.
 124 .TP
 125 .B "\-L \fIlabel\fP"
 126 Use the partition that has the specified
 127 .IR label .
 128 (For this, access to
 129 .I /proc/partitions
 130 is needed.)
 131 .TP
 132 .B "\-p, \-\-priority \fIpriority\fP"
 133 Specify the priority of the swap device.
 134 .I priority
 135 is a value between \-1 and 32767.  Higher numbers indicate
 136 higher priority.  See
 137 .BR swapon (2)
 138 for a full description of swap priorities. Add
 139 .BI pri= value
 140 to the option field of
 141 .I /etc/fstab
 142 for use with
 143 .BR "swapon -a" .
 144 When priority is not defined it defaults to \-1.
 145 .TP
 146 .B "\-s, \-\-summary"
 147 Display swap usage summary by device. Equivalent to "cat /proc/swaps".
 148 Not available before Linux 2.1.25.
 149 .TP
 150 \fB\-\-show\fR [\fIcolumn\fR, ...]
 151 Display definable device table similar to
 152 .B \-\-summary
 153 output.  See \-\-help output for
 154 .I column
 155 list.
 156 .TP
 157 .B \-\-noheadings
 158 Do not print headings when displaying
 159 .B \-\-show
 160 output.
 161 .TP
 162 .B \-\-raw
 163 Display
 164 .B \-\-show
 165 output without aligning table columns.
 166 .TP
 167 .B \-\-bytes
 168 Display swap size in bytes in
 169 .B \-\-show
 170 output instead of user friendly size and unit.
 171 .B "\-U \fIuuid\fP"
 172 Use the partition that has the specified
 173 .IR uuid .
 174 .TP
 175 .B "\-v, \-\-verbose"
 176 Be verbose.
 177 .TP
 178 .B "\-V, \-\-version"
 179 Display version information and exit.
 180 .SH NOTES
 181 You should not use
 182 .B swapon
 183 on a file with holes.
 184 Swap over NFS may not work.
 185 .PP
 186 .B swapon
 187 automatically detects and rewrites swap space signature with old software
 188 suspend data (e.g S1SUSPEND, S2SUSPEND, ...). The problem is that if we don't
 189 do it, then we get data corruption the next time an attempt at unsuspending is
 190 made.
 191 .PP
 192 .B swapon
 193 may not work correctly when using a swap file with some versions of btrfs.
 194 This is due to the swap file implementation in the kernel expecting to be able
 195 to write to the file directly, without the assistance of the file system.
 196 Since btrfs is a copy-on-write file system, the file location may not be
 197 static and corruption can result. Btrfs actively disallows the use of files
 198 on its file systems by refusing to map the file. This can be seen in the system
 199 log as "swapon: swapfile has holes." One possible workaround is to map the
 200 file to a loopback device. This will allow the file system to determine the
 201 mapping properly but may come with a performance impact.
 202
 203 .SH ENVIRONMENT
 204 .IP LIBMOUNT_DEBUG=0xffff
 205 enables debug output.
 206
 207 .SH SEE ALSO
 208 .BR swapon (2),
 209 .BR swapoff (2),
 210 .BR fstab (5),
 211 .BR init (8),
 212 .BR mkswap (8),
 213 .BR rc (8),
 214 .BR mount (8)
 215 .SH FILES
 216 .br
 217 .I /dev/sd??
 218 standard paging devices
 219 .br
 220 .I /etc/fstab
 221 ascii filesystem description table
 222 .SH HISTORY
 223 The
 224 .B swapon
 225 command appeared in 4.0BSD.
 226 .SH AVAILABILITY
 227 The swapon command is part of the util-linux package and is available from
 228 ftp://ftp.kernel.org/pub/linux/utils/util-linux/.