254 lines
7.5 KiB
Groff
254 lines
7.5 KiB
Groff
.\" Copyright (c) 1980, 1991 Regents of the University of California.
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)swapon.8 6.3 (Berkeley) 3/16/91
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.\"
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.TH SWAPON 8 "October 2014" "util-linux" "System Administration"
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.SH NAME
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swapon, swapoff \- enable/disable devices and files for paging and swapping
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.SH SYNOPSIS
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.BR swapon
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[options]
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.RI [ specialfile ...]
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.br
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.B swapoff
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.RB [ \-va ]
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.RI [ specialfile ...]
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.SH DESCRIPTION
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.B swapon
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is used to specify devices on which paging and swapping are to take place.
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The device or file used is given by the
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.I specialfile
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parameter. It may be of the form
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.BI \-L " label"
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or
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.BI \-U " uuid"
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to indicate a device by label or uuid.
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Calls to
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.B swapon
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normally occur in the system boot scripts making all swap devices available, so
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that the paging and swapping activity is interleaved across several devices and
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files.
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.B swapoff
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disables swapping on the specified devices and files.
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When the
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.B \-a
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flag is given, swapping is disabled on all known swap devices and files
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(as found in
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.I /proc/swaps
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or
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.IR /etc/fstab ).
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.SH OPTIONS
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.TP
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.BR \-a , " \-\-all"
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All devices marked as ``swap'' in
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.I /etc/fstab
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are made available, except for those with the ``noauto'' option.
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Devices that are already being used as swap are silently skipped.
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.TP
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.BR \-d , " \-\-discard" [ =\fIpolicy\fR]
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Enable swap discards, if the swap backing device supports the discard or
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trim operation. This may improve performance on some Solid State Devices,
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but often it does not. The option allows one to select between two
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available swap discard policies:
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.BI \-\-discard=once
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to perform a single-time discard operation for the whole swap area at swapon;
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or
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.BI \-\-discard=pages
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to asynchronously discard freed swap pages before they are avaliable for reuse.
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If no policy is selected, the default behavior is to enable both discard types.
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The
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.I /etc/fstab
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mount options
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.BR discard ,
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.BR discard=once ,
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or
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.B discard=pages
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may also be used to enable discard flags.
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.TP
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.BR \-e , " \-\-ifexists"
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Silently skip devices that do not exist.
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The
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.I /etc/fstab
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mount option
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.B nofail
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may also be used to skip non-existing device.
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.TP
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.BR \-f , " \-\-fixpgsz"
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Reinitialize (exec mkswap) the swap space if its page size does not
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match that of the current running kernel.
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.BR mkswap (2)
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initializes the whole device and does not check for bad blocks.
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.TP
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.BR \-h , " \-\-help"
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Display help text and exit.
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.TP
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.BI \-L " label"
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Use the partition that has the specified
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.IR label .
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(For this, access to
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.I /proc/partitions
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is needed.)
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.TP
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.BR \-o , " \-\-options " \fIopts\fP
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Specify swap options by an fstab-compatible comma-separated string.
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For example:
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.RS
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.RS
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.sp
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.B "swapon -o pri=1,discard=pages,nofail /dev/sda2"
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.sp
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.RE
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The \fIopts\fP string is evaluated last and overrides all other
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command line options.
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.RE
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.TP
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.BR \-p , " \-\-priority " \fIpriority\fP
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Specify the priority of the swap device.
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.I priority
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is a value between \-1 and 32767. Higher numbers indicate
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higher priority. See
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.BR swapon (2)
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for a full description of swap priorities. Add
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.BI pri= value
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to the option field of
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.I /etc/fstab
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for use with
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.BR "swapon -a" .
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When no priority is defined, it defaults to \-1.
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.TP
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.BR \-s , " \-\-summary"
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Display swap usage summary by device. Equivalent to "cat /proc/swaps".
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Not available before Linux 2.1.25. This output format is DEPRECATED in favour
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of \fB\-\-show\fR that provides better control on output data.
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.TP
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.BR \-\-show [ =\fIcolumn\fR ...]
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Display a definable table of swap areas. See the
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.B \-\-help
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output for a list of available columns.
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.TP
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.B \-\-noheadings
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Do not print headings when displaying
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.B \-\-show
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output.
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.TP
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.B \-\-raw
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Display
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.B \-\-show
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output without aligning table columns.
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.TP
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.B \-\-bytes
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Display swap size in bytes in
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.B \-\-show
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output instead of in user-friendly units.
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.TP
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.BI \-U " uuid"
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Use the partition that has the specified
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.IR uuid .
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.TP
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.BR \-v , " \-\-verbose"
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Be verbose.
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.TP
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.BR \-V , " \-\-version"
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Display version information and exit.
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.SH NOTES
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You should not use \fBswapon\fR on a file with holes.
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This can be seen in the system log as
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.RS
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.sp
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.B "swapon: swapfile has holes."
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.sp
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.RE
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The swap file implementation in the kernel expects to be able to write to the
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file directly, without the assistance of the filesystem. This is a problem on
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preallocated files (e.g.
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.BR fallocate (1))
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on filesystems like \fBXFS\fR or \fBext4\fR, and on copy-on-write
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filesystems like \fBbtrfs\fR.
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.PP
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It is recommended to use
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.BR dd (1)
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and
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.I /dev/zero
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to avoid holes on XFS and ext4.
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.PP
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.B swapon
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may not work correctly when using a swap file with some versions of
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\fBbtrfs\fR. This is due to btrfs being a copy-on-write filesystem: the
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file location may not be static and corruption can result. Btrfs actively
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disallows the use of swap files on its filesystems by refusing to map the file.
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.PP
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One possible workaround is to map the swap
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file to a loopback device. This will allow the filesystem to determine the
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mapping properly but may come with a performance impact.
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.PP
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Swap over \fBNFS\fR may not work.
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.PP
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.B swapon
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automatically detects and rewrites a swap space signature with old software
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suspend data (e.g S1SUSPEND, S2SUSPEND, ...). The problem is that if we don't
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do it, then we get data corruption the next time an attempt at unsuspending is
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made.
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.SH ENVIRONMENT
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.IP LIBMOUNT_DEBUG=all
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enables libmount debug output.
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.IP LIBBLKID_DEBUG=all
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enables libblkid debug output.
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.SH SEE ALSO
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.BR swapon (2),
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.BR swapoff (2),
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.BR fstab (5),
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.BR init (8),
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.BR mkswap (8),
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.BR rc (8),
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.BR mount (8)
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.SH FILES
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.br
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.I /dev/sd??
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standard paging devices
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.br
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.I /etc/fstab
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ascii filesystem description table
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.SH HISTORY
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The
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.B swapon
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command appeared in 4.0BSD.
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.SH AVAILABILITY
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The swapon command is part of the util-linux package and is available from
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ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
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