path: root/sys-utils/swapon.8

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.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/.