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+.\" Copyright (c) 1980, 1991 Regents of the University of California.
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed by the University of
+.\"	California, Berkeley and its contributors.
+.\" 4. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"     @(#)swapon.8	6.3 (Berkeley) 3/16/91
+.\"
+.TH SWAPON 8 "October 2014" "util-linux" "System Administration"
+.SH NAME
+swapon, swapoff \- enable/disable devices and files for paging and swapping
+.SH SYNOPSIS
+.B swapon
+[options]
+.RI [ specialfile ...]
+.br
+.B swapoff
+.RB [ \-va ]
+.RI [ specialfile ...]
+.SH DESCRIPTION
+.B swapon
+is used to specify devices on which paging and swapping are to take place.
+
+The device or file used is given by the
+.I specialfile
+parameter.  It may be of the form
+.BI \-L " label"
+or
+.BI \-U " uuid"
+to indicate a device by label or uuid.
+
+Calls to
+.B swapon
+normally occur in the system boot scripts making all swap devices available, so
+that the paging and swapping activity is interleaved across several devices and
+files.
+
+.B swapoff
+disables swapping on the specified devices and files.
+When the
+.B \-a
+flag is given, swapping is disabled on all known swap devices and files
+(as found in
+.I /proc/swaps
+or
+.IR /etc/fstab ).
+
+.SH OPTIONS
+.TP
+.BR \-a , " \-\-all"
+All devices marked as ``swap'' in
+.I /etc/fstab
+are made available, except for those with the ``noauto'' option.
+Devices that are already being used as swap are silently skipped.
+.TP
+.BR \-d , " \-\-discard" [ =\fIpolicy\fR]
+Enable swap discards, if the swap backing device supports the discard or
+trim operation.  This may improve performance on some Solid State Devices,
+but often it does not.  The option allows one to select between two
+available swap discard policies:
+.B \-\-discard=once
+to perform a single-time discard operation for the whole swap area at swapon;
+or
+.B \-\-discard=pages
+to asynchronously discard freed swap pages before they are available for reuse.
+If no policy is selected, the default behavior is to enable both discard types.
+The
+.I /etc/fstab
+mount options
+.BR discard ,
+.BR discard=once ,
+or
+.B discard=pages
+may also be used to enable discard flags.
+.TP
+.BR \-e , " \-\-ifexists"
+Silently skip devices that do not exist.
+The
+.I /etc/fstab
+mount option
+.B nofail
+may also be used to skip non-existing device.
+
+.TP
+.BR \-f , " \-\-fixpgsz"
+Reinitialize (exec mkswap) the swap space if its page size does not
+match that of the current running kernel.
+.BR mkswap (2)
+initializes the whole device and does not check for bad blocks.
+.TP
+.BR \-h , " \-\-help"
+Display help text and exit.
+.TP
+.BI \-L " label"
+Use the partition that has the specified
+.IR label .
+(For this, access to
+.I /proc/partitions
+is needed.)
+.TP
+.BR \-o , " \-\-options " \fIopts\fP
+Specify swap options by an fstab-compatible comma-separated string.
+For example:
+.RS
+.RS
+.sp
+.B "swapon -o pri=1,discard=pages,nofail /dev/sda2"
+.sp
+.RE
+The \fIopts\fP string is evaluated last and overrides all other
+command line options.
+.RE
+.TP
+.BR \-p , " \-\-priority " \fIpriority\fP
+Specify the priority of the swap device.
+.I priority
+is a value between \-1 and 32767.  Higher numbers indicate
+higher priority.  See
+.BR swapon (2)
+for a full description of swap priorities.  Add
+.BI pri= value
+to the option field of
+.I /etc/fstab
+for use with
+.BR "swapon -a" .
+When no priority is defined, it defaults to \-1.
+.TP
+.BR \-s , " \-\-summary"
+Display swap usage summary by device.  Equivalent to "cat /proc/swaps".
+This output format is DEPRECATED in favour
+of \fB\-\-show\fR that provides better control on output data.
+.TP
+.BR \-\-show [ =\fIcolumn\fR ...]
+Display a definable table of swap areas.  See the
+.B \-\-help
+output for a list of available columns.
+.TP
+.B \-\-output\-all
+Output all available columns.
+.TP
+.B \-\-noheadings
+Do not print headings when displaying
+.B \-\-show
+output.
+.TP
+.B \-\-raw
+Display
+.B \-\-show
+output without aligning table columns.
+.TP
+.B \-\-bytes
+Display swap size in bytes in
+.B \-\-show
+output instead of in user-friendly units.
+.TP
+.BI \-U  " uuid"
+Use the partition that has the specified
+.IR uuid .
+.TP
+.BR \-v , " \-\-verbose"
+Be verbose.
+.TP
+.BR \-V , " \-\-version"
+Display version information and exit.
+.SH NOTES
+You should not use \fBswapon\fR on a file with holes.
+This can be seen in the system log as
+.RS
+.sp
+.B "swapon: swapfile has holes."
+.sp
+.RE
+The swap file implementation in the kernel expects to be able to write to the
+file directly, without the assistance of the filesystem.  This is a problem on
+preallocated files (e.g.
+.BR fallocate (1))
+on filesystems like \fBXFS\fR or \fBext4\fR, and on copy-on-write
+filesystems like \fBbtrfs\fR.
+.PP
+It is recommended to use
+.BR dd (1)
+and
+.I /dev/zero
+to avoid holes on XFS and ext4.
+.PP
+.B swapon
+may not work correctly when using a swap file with some versions of
+\fBbtrfs\fR.  This is due to btrfs being a copy-on-write filesystem: the
+file location may not be static and corruption can result.  Btrfs actively
+disallows the use of swap files on its filesystems by refusing to map the file.
+.PP
+One possible workaround is to map the swap
+file to a loopback device.  This will allow the filesystem to determine the
+mapping properly but may come with a performance impact.
+.PP
+Swap over \fBNFS\fR may not work.
+.PP
+.B swapon
+automatically detects and rewrites a swap space signature with old software
+suspend data (e.g. S1SUSPEND, S2SUSPEND, ...). The problem is that if we don't
+do it, then we get data corruption the next time an attempt at unsuspending is
+made.
+
+.SH ENVIRONMENT
+.IP LIBMOUNT_DEBUG=all
+enables libmount debug output.
+.IP LIBBLKID_DEBUG=all
+enables libblkid debug output.
+
+.SH SEE ALSO
+.BR swapoff (2),
+.BR swapon (2),
+.BR fstab (5),
+.BR init (8),
+.BR mkswap (8),
+.BR mount (8),
+.BR rc (8)
+.SH FILES
+.br
+.I /dev/sd??
+standard paging devices
+.br
+.I /etc/fstab
+ascii filesystem description table
+.SH HISTORY
+The
+.B swapon
+command appeared in 4.0BSD.
+.SH AVAILABILITY
+The swapon command is part of the util-linux package and is available from
+https://www.kernel.org/pub/linux/utils/util-linux/.