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swapon, swapoff — enable/disable devices and files for paging and swapping
swapon [options] [
specialfile... ]
swapoff [−va] [ specialfile... ]
swapon is used to specify devices on which paging and swapping are to take place.
The device or file used is given by the specialfile parameter. It may
be of the form −L
label or −U uuid to indicate a device by label
or uuid.
Calls to 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.
swapoff
disables swapping on the specified devices and files. When
the −a flag is given,
swapping is disabled on all known swap devices and files (as
found in /proc/swaps or
/etc/fstab).
−a,
−−allAll devices marked as ``swap'' in /etc/fstab are made available, except
for those with the ``noauto'' option. Devices that are
already being used as swap are silently skipped.
−d,
−−discard [=policy]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:
−−discard=once
to perform a single-time discard operation for the
whole swap area at swapon; or −−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
/etc/fstab mount options
discard,
discard=once,
or discard=pages may be
also used to enable discard flags.
−e,
−−ifexistsSilently skip devices that do not exist. The
/etc/fstab mount option
nofail
may be also used to skip non-existing device.
−f,
−−fixpgszReinitialize (exec /sbin/mkswap) the swap space if its page size does not match that of the current running kernel. mkswap(2) initializes the whole device and does not check for bad blocks.
−h,−−helpDisplay help text and exit.
−L
labelUse the partition that has the specified label. (For this,
access to /proc/partitions is needed.)
−p,
−−priority prioritySpecify the priority of the swap device.
priority
is a value between −1 and 32767. Higher numbers
indicate higher priority. See swapon(2) for a full
description of swap priorities. Add pri=value to the option
field of /etc/fstab for
use with swapon
-a. When priority is not defined it
defaults to −1.
−s,
−−summaryDisplay swap usage summary by device. Equivalent to "cat /proc/swaps". Not available before Linux 2.1.25.
−−show [column,
...]Display definable device table similar to
−−summary
output. See −−help output for column list.
−−noheadingsDo not print headings when displaying −−show output.
−−rawDisplay −−show output without
aligning table columns.
−−bytesDisplay swap size in bytes in −−show output instead of
user friendly size and unit. −U Use the
partition that has the specified uuiduuid.
−v,
−−verboseBe verbose.
−V,
−−versionDisplay version information and exit.
You should not use swapon on a file with holes. Swap over NFS may not work.
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.
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.
The swapon command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
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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 Sun Dec 27 12:31:30 1992: Modified by faithcs.unc.edu Sat Mar 6 20:46:02 1993: Modified by faithcs.unc.edu Sat Oct 9 09:35:30 1993: Converted to man format by faithcs.unc.edu Sat Nov 27 20:22:42 1993: Updated authorship information, faithcs.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 |