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.\"     @(#)swapon.8	6.3 (Berkeley) 3/16/91
.\"
.\" Sun Dec 27 12:31:30 1992: Modified by faith@cs.unc.edu
.\" Sat Mar  6 20:46:02 1993: Modified by faith@cs.unc.edu
.\" Sat Oct  9 09:35:30 1993: Converted to man format by faith@cs.unc.edu
.\" Sat Nov 27 20:22:42 1993: Updated authorship information, faith@cs.unc.edu
.\" Mon Sep 25 14:12:38 1995: Added -v and -p information
.\" Tue Apr 30 03:32:07 1996: Added some text from A. Koppenhoefer
.\"
.TH SWAPON 8 "July 2014" "util-linux" "System Administration"
.SH NAME
swapon, swapoff \- enable/disable devices and files for paging and swapping
.SH SYNOPSIS
.BR 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:
.BI \-\-discard=once
to perform a single-time discard operation for the whole swap area at swapon;
or
.BI \-\-discard=pages
to discard freed swap pages before they are reused, while swapping.
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 /sbin/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 \-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 priority is not defined it defaults to \-1.
.TP
.BR \-s , " \-\-summary"
Display swap usage summary by device.  Equivalent to "cat /proc/swaps".
Not available before Linux 2.1.25.  This output format is DEPRECATED in favour
of \fB\-\-show\fR that provides better control on output data.
.TP
.BR \-\-show " [\fIcolumn\fR, ...]"
Display definable device table similar to
.B \-\-summary
output.  See
.B \-\-help
output for a list of 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
.B "\-v, \-\-verbose"
Be verbose.
.TP
.B "\-V, \-\-version"
Display version information and exit.
.SH NOTES
You should not use
.B swapon
on a file with holes.
Swap over NFS may not work.
.PP
.B swapon
automatically detects and rewrites 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.
.PP
.B swapon
may not work correctly when using a swap file with some versions of btrfs.
This is due to the swap file implementation in the kernel expecting to be able
to write to the file directly, without the assistance of the file system.
Since btrfs is a copy-on-write file system, the file location may not be
static and corruption can result.  Btrfs actively disallows the use of files
on its file systems by refusing to map the file.  This can be seen in the system
log as "swapon: swapfile has holes."  One possible workaround is to map the
file to a loopback device.  This will allow the file system to determine the
mapping properly but may come with a performance impact.

.SH ENVIRONMENT
.IP LIBMOUNT_DEBUG=0xffff
enables debug output.

.SH SEE ALSO
.BR swapon (2),
.BR swapoff (2),
.BR fstab (5),
.BR init (8),
.BR mkswap (8),
.BR rc (8),
.BR mount (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
ftp://ftp.kernel.org/pub/linux/utils/util-linux/.