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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 "25 September 1995" "Linux 1.x" "Linux Programmer's Manual"
.SH NAME
swapon, swapoff \- enable/disable devices and files for paging and swapping
.SH SYNOPSIS
Get info:
.br
.in +5
.B swapon \-s
.RB [ \-h ]
.RB [ \-V ]
.sp
.in -5
Enable/disable:
.br
.in +5
.B swapon
.RB [ \-f ]
.RB [ \-p
.IR priority ]
.RB [ \-v ]
.IR specialfile ...
.br
.B swapoff
.RB [ \-v ]
.IR specialfile ...
.sp
.in -5
Enable/disable all:
.br
.in +5
.B swapon \-a
.RB [ \-e ]
.RB [ \-f ]
.RB [ \-v ]
.br
.B swapoff \-a
.RB [ \-v ]
.in -5
.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 ).

.TP
.B "\-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
.B "\-d, \-\-discard\fR [=\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
.BI discard,
.BI discard=once,
or
.BI discard=pages
may be also used to enable discard flags.
.TP
.B "\-e, \-\-ifexists"
Silently skip devices that do not exist.
.TP
.B "\-f, \-\-fixpgsz"
Reinitialize (exec /sbin/mkswap) the swap space if its page size does not
match that of the the current running kernel.
.B mkswap(2)
initializes the whole device and does not check for bad blocks.
.TP
.B \-h, \-\-help
Provide help.
.TP
.B "\-L \fIlabel\fP"
Use the partition that has the specified
.IR label .
(For this, access to
.I /proc/partitions
is needed.)
.TP
.B "\-p, \-\-priority \fIpriority\fP"
Specify the priority of the swap device.
.I priority
is a value between 0 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" .
.TP
.B "\-s, \-\-summary"
Display swap usage summary by device. Equivalent to "cat /proc/swaps".
Not available before Linux 2.1.25.
.TP
.B "\-U \fIuuid\fP"
Use the partition that has the specified
.IR uuid .
.TP
.B "\-v, \-\-verbose"
Be verbose.
.TP
.B "\-V, \-\-version"
Display version.
.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.
.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-ng package and is available from
ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.
