Manual pages: order NOTES / HISTORY / BUGS / EXAMPLE consistently

[thirdparty/util-linux.git] / sys-utils / swapon.8

.\" 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 (8)
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 ENVIRONMENT
.IP LIBMOUNT_DEBUG=all
enables libmount debug output.
.IP LIBBLKID_DEBUG=all
enables libblkid debug output.

.SH FILES
.br
.I /dev/sd??
standard paging devices
.br
.I /etc/fstab
ascii filesystem description table
.SH NOTES
.SS Files with holes
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
files with holes or on copy-on-write files on filesystems like Btrfs.
.sp
Commands like
.BR cp (1)
or
.BR truncate (1)
create files with holes.  These files will be rejected by swapon.
.sp
Preallocated files created by
.BR fallocate (1)
may be interpreted as files with holes too depending of the filesystem.
Preallocated swap files are supported on XFS since Linux 4.18.
.sp
The most portable solution to create a swap file is to use
.BR dd (1)
and /dev/zero.
.SS Btrfs
Swap files on Btrfs are supported since Linux 5.0 on files with nocow attribute.
See the
.BR btrfs (5)
manual page for more details.
.SS NFS
Swap over \fBNFS\fR may not work.
.SS Suspend
.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 HISTORY
The
.B swapon
command appeared in 4.0BSD.
.SH SEE ALSO
.BR swapoff (2),
.BR swapon (2),
.BR fstab (5),
.BR init (8),
.BR fallocate (1),
.BR mkswap (8),
.BR mount (8),
.BR rc (8)
.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/.