sys-utils/swapon.8

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  32 .\"     @(#)swapon.8    6.3 (Berkeley) 3/16/91
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  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 .B 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 .B \-\-discard=once
  87 to perform a single-time discard operation for the whole swap area at swapon;
  88 or
  89 .B \-\-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 (8)
 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 \-\-output\-all
 164 Output all available columns.
 165 .TP
 166 .B \-\-noheadings
 167 Do not print headings when displaying
 168 .B \-\-show
 169 output.
 170 .TP
 171 .B \-\-raw
 172 Display
 173 .B \-\-show
 174 output without aligning table columns.
 175 .TP
 176 .B \-\-bytes
 177 Display swap size in bytes in
 178 .B \-\-show
 179 output instead of in user-friendly units.
 180 .TP
 181 .BI \-U  " uuid"
 182 Use the partition that has the specified
 183 .IR uuid .
 184 .TP
 185 .BR \-v , " \-\-verbose"
 186 Be verbose.
 187 .TP
 188 .BR \-V , " \-\-version"
 189 Display version information and exit.
 190 .SH ENVIRONMENT
 191 .IP LIBMOUNT_DEBUG=all
 192 enables libmount debug output.
 193 .IP LIBBLKID_DEBUG=all
 194 enables libblkid debug output.
 195
 196 .SH FILES
 197 .br
 198 .I /dev/sd??
 199 standard paging devices
 200 .br
 201 .I /etc/fstab
 202 ascii filesystem description table
 203 .SH NOTES
 204 .SS Files with holes
 205 The swap file implementation in the kernel expects to be able to write to the
 206 file directly, without the assistance of the filesystem.  This is a problem on
 207 files with holes or on copy-on-write files on filesystems like Btrfs.
 208 .sp
 209 Commands like
 210 .BR cp (1)
 211 or
 212 .BR truncate (1)
 213 create files with holes.  These files will be rejected by swapon.
 214 .sp
 215 Preallocated files created by
 216 .BR fallocate (1)
 217 may be interpreted as files with holes too depending of the filesystem.
 218 Preallocated swap files are supported on XFS since Linux 4.18.
 219 .sp
 220 The most portable solution to create a swap file is to use
 221 .BR dd (1)
 222 and /dev/zero.
 223 .SS Btrfs
 224 Swap files on Btrfs are supported since Linux 5.0 on files with nocow attribute.
 225 See the
 226 .BR btrfs (5)
 227 manual page for more details.
 228 .SS NFS
 229 Swap over \fBNFS\fR may not work.
 230 .SS Suspend
 231 .B swapon
 232 automatically detects and rewrites a swap space signature with old software
 233 suspend data (e.g., S1SUSPEND, S2SUSPEND, ...). The problem is that if we don't
 234 do it, then we get data corruption the next time an attempt at unsuspending is
 235 made.
 236 .SH HISTORY
 237 The
 238 .B swapon
 239 command appeared in 4.0BSD.
 240 .SH SEE ALSO
 241 .BR swapoff (2),
 242 .BR swapon (2),
 243 .BR fstab (5),
 244 .BR init (8),
 245 .BR fallocate (1),
 246 .BR mkswap (8),
 247 .BR mount (8),
 248 .BR rc (8)
 249 .SH AVAILABILITY
 250 The swapon command is part of the util-linux package and is available from
 251 https://www.kernel.org/pub/linux/utils/util-linux/.