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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.
192 The swap file implementation in the kernel expects to be able to write to the file directly,
193 without the assistance of the filesystem. This is a problem on files with holes
194 or on copy-on-write files on filesystems like btrfs.
200 create files with holes. These files will be rejected by swapon.
202 In some cases preallocated files (e.g. fallocate(1) on XFS) maybe interpreted
203 by old kernel versions as files with holes too. The preallocated swap files on XFS are
204 supported since Linux 4.18.
206 The most portable solution to create a swap file is to use
208 and /dev/zero in these cases.
210 The swap files on btrfs are supported since Linux 5.0 on files with nocow and
211 nodatasum attributes and without compression. The previous versions may use
212 copy-on-write semantic for the swap files, it means that the file location is
213 may not be static and corruption can result.
215 One possible workaround is to map the swap file to a loopback device. This will allow the filesystem
216 to determine the mapping properly but may come with a performance impact.
218 Swap over \fBNFS\fR may not work.
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
226 .IP LIBMOUNT_DEBUG=all
227 enables libmount debug output.
228 .IP LIBBLKID_DEBUG=all
229 enables libblkid debug output.
242 standard paging devices
245 ascii filesystem description table
249 command appeared in 4.0BSD.
251 The swapon command is part of the util-linux package and is available from
252 https://www.kernel.org/pub/linux/utils/util-linux/.