swapon(8) [freebsd man page]
SWAPON(8) BSD System Manager's Manual SWAPON(8) NAME
swapon, swapoff, swapctl -- specify devices for paging and swapping SYNOPSIS
swapon [-F fstab] -aLq | file ... swapoff [-F fstab] -aLq | file ... swapctl [-AghklmsU] [-a file ... | -d file ...] DESCRIPTION
The swapon, swapoff and swapctl utilities are used to control swap devices in the system. At boot time all swap entries in /etc/fstab are added automatically when the system goes multi-user. Swap devices use a fixed interleave; the maximum number of devices is unlimited. There is no priority mechanism. The swapon utility adds the specified swap devices to the system. If the -a option is used, all swap devices in /etc/fstab will be added, unless their ``noauto'' or ``late'' option is also set. If the -L option is specified, swap devices with the ``late'' option will be added as well as ones with no option. If the -q option is used, informational messages will not be written to standard output when a swap device is added. The swapoff utility removes the specified swap devices from the system. If the -a option is used, all swap devices in /etc/fstab will be removed, unless their ``noauto'' or ``late'' option is also set. If the -L option is specified, swap devices with the ``late'' option will be removed as well as ones with no option. If the -q option is used, informational messages will not be written to standard output when a swap device is removed. Note that swapoff will fail and refuse to remove a swap device if there is insufficient VM (memory + remaining swap devices) to run the system. The swapoff utility must move swapped pages out of the device being removed which could lead to high system loads for a period of time, depending on how much data has been swapped out to that device. Other options supported by both swapon and swapoff are as follows: -F fstab Specify the fstab file to use. The swapctl utility exists primarily for those familiar with other BSDs and may be used to add, remove, or list swap devices. Note that the -a option is used differently in swapctl and indicates that a specific list of devices should be added. The -d option indicates that a spe- cific list should be removed. The -A and -U options to swapctl operate on all swap entries in /etc/fstab which do not have their ``noauto'' option set. Swap information can be generated using the swapinfo(8) utility, pstat -s, or swapctl -l. The swapctl utility has the following options for listing swap: -h Output values in human-readable form. -g Output values in gigabytes. -k Output values in kilobytes. -m Output values in megabytes. -l List the devices making up system swap. -s Print a summary line for system swap. The BLOCKSIZE environment variable is used if not specifically overridden. 512 byte blocks are used by default. FILES
/dev/{ada,da}?s?b standard paging devices /dev/md? memory disk devices /etc/fstab ASCII file system description table DIAGNOSTICS
These utilities may fail for the reasons described in swapon(2). SEE ALSO
swapon(2), fstab(5), init(8), mdconfig(8), pstat(8), rc(8) HISTORY
The swapon utility appeared in 4.0BSD. The swapoff and swapctl utilities appeared in FreeBSD 5.1. BSD
November 22, 2013 BSD
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SWAPON(8) System Administration SWAPON(8) NAME
swapon, swapoff - enable/disable devices and files for paging and swapping SYNOPSIS
Get info: swapon -s [-h] [-V] Enable/disable: swapon [-d] [-f] [-p priority] [-v] specialfile... swapoff [-v] specialfile... Enable/disable all: swapon -a [-e] [-f] [-v] swapoff -a [-v] DESCRIPTION
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, --all All 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 Discard freed swap pages before they are reused, if the swap device supports the discard or trim operation. This may improve per- formance on some Solid State Devices, but often it does not. The /etc/fstab mount option discard may be also used to enable discard flag. -e, --ifexists Silently skip devices that do not exist. The /etc/fstab mount option nofail may be also used to skip non-existing device. -f, --fixpgsz Reinitialize (exec /sbin/mkswap) the swap space if its page size does not match that of the the current running kernel. mkswap(2) initializes the whole device and does not check for bad blocks. -h, --help Provide help. -L label Use the partition that has the specified label. (For this, access to /proc/partitions is needed.) -p, --priority priority Specify the priority of the swap device. priority is a value between 0 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. -s, --summary Display swap usage summary by device. Equivalent to "cat /proc/swaps". Not available before Linux 2.1.25. -U uuid Use the partition that has the specified uuid. -v, --verbose Be verbose. -V, --version Display version. NOTES
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 ker- nel 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. SEE ALSO
swapon(2), swapoff(2), fstab(5), init(8), mkswap(8), rc(8), mount(8) FILES
/dev/sd?? standard paging devices /etc/fstab ascii filesystem description table HISTORY
The swapon command appeared in 4.0BSD. AVAILABILITY
The swapon command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/. util-linux September 1995 SWAPON(8)