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32 .\" @(#)swapon.8 6.3 (Berkeley) 3/16/91
34 .TH SWAPON 8 "October 2014" "util-linux" "System Administration"
36 swapon, swapoff \- enable/disable devices and files for paging and swapping
40 .RI [ specialfile ...]
44 .RI [ specialfile ...]
47 is used to specify devices on which paging and swapping are to take place.
49 The device or file used is given by the
51 parameter. It may be of the form
55 to indicate a device by label or uuid.
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
64 disables swapping on the specified devices and files.
67 flag is given, swapping is disabled on all known swap devices and files
76 All devices marked as ``swap'' in
78 are made available, except for those with the ``noauto'' option.
79 Devices that are already being used as swap are silently skipped.
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:
87 to perform a single-time discard operation for the whole swap area at swapon;
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.
99 may also be used to enable discard flags.
101 .BR \-e , " \-\-ifexists"
102 Silently skip devices that do not exist.
107 may also be used to skip non-existing device.
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.
114 initializes the whole device and does not check for bad blocks.
116 .BR \-h , " \-\-help"
117 Display help text and exit.
120 Use the partition that has the specified
126 .BR \-o , " \-\-options " \fIopts\fP
127 Specify swap options by an fstab-compatible comma-separated string.
132 .B "swapon -o pri=1,discard=pages,nofail /dev/sda2"
135 The \fIopts\fP string is evaluated last and overrides all other
136 command line options.
139 .BR \-p , " \-\-priority " \fIpriority\fP
140 Specify the priority of the swap device.
142 is a value between \-1 and 32767. Higher numbers indicate
145 for a full description of swap priorities. Add
147 to the option field of
151 When no priority is defined, it defaults to \-1.
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.
158 .BR \-\-show [ =\fIcolumn\fR ...]
159 Display a definable table of swap areas. See the
161 output for a list of available columns.
164 Output all available columns.
167 Do not print headings when displaying
174 output without aligning table columns.
177 Display swap size in bytes in
179 output instead of in user-friendly units.
182 Use the partition that has the specified
185 .BR \-v , " \-\-verbose"
188 .BR \-V , " \-\-version"
189 Display version information and exit.
191 You should not use \fBswapon\fR on a file with holes.
192 This can be seen in the system log as
195 .B "swapon: swapfile has holes."
198 The swap file implementation in the kernel expects to be able to write to the
199 file directly, without the assistance of the filesystem. This is a problem on
200 preallocated files (e.g.
202 on filesystems like \fBXFS\fR or \fBext4\fR, and on copy-on-write
203 filesystems like \fBbtrfs\fR.
205 It is recommended to use
209 to avoid holes on XFS and ext4.
212 may not work correctly when using a swap file with some versions of
213 \fBbtrfs\fR. This is due to btrfs being a copy-on-write filesystem: the
214 file location may not be static and corruption can result. Btrfs actively
215 disallows the use of swap files on its filesystems by refusing to map the file.
217 One possible workaround is to map the swap
218 file to a loopback device. This will allow the filesystem to determine the
219 mapping properly but may come with a performance impact.
221 Swap over \fBNFS\fR may not work.
224 automatically detects and rewrites a swap space signature with old software
225 suspend data (e.g. S1SUSPEND, S2SUSPEND, ...). The problem is that if we don't
226 do it, then we get data corruption the next time an attempt at unsuspending is
230 .IP LIBMOUNT_DEBUG=all
231 enables libmount debug output.
232 .IP LIBBLKID_DEBUG=all
233 enables libblkid debug output.
246 standard paging devices
249 ascii filesystem description table
253 command appeared in 4.0BSD.
255 The swapon command is part of the util-linux package and is available from
256 https://www.kernel.org/pub/linux/utils/util-linux/.