dphys-swapfile(8) [debian man page]
DPHYS-SWAPFILE(8) System Manager's Manual DPHYS-SWAPFILE(8) NAME
dphys-swapfile - set up, mount/unmount, and delete an swap file SYNOPSIS
dphys-swapfile setup|swapon|swapoff|uninstall DESCRIPTION
dphys-swapfile computes the size for an optimal swap file (and resizes an existing swap file if necessary), mounts an swap file, unmounts it, and and delete it if not wanted any more. OPTIONS
There is only one parameter, an command, which can be either of these: setup Tells dphys-swapfile to compute the optimal swap file size and (re-)generate an fitting swap file. Default it 2 times RAM size. This can be called at boot time, so the file allways stays the right size for current RAM, or run by hand whenever RAM size has changed. swapon and swapoff These run the swapon and swapoff commands on the swapfile. Note that direct swapon/off from /etc/fstab is not possible, as that is (at least on Debian) done in the same script that mounts /var (which is where the swap file most likely resides). And we need to do our setup between those actions. So pass up /etc/fstab, and do our own swapon/off. uninstall Gets rid of an unwanted swap file, reclaiming the disk space. CONFIG
The config file /etc/dphys-swapfile allows the user to set up the working environment for dphys-swapfile. This config file is a sh script fragment full of assignments, which is sourced. Standard sh syntax rules apply. Assignments are: CONF_SWAPFILE Set where the swap file should be placed. Defaults to /var/swap. It is unlikely that you will need to change this, unless you have very strange partitioning, and then you will most likely be using an swap partition anyway. CONF_SWAPSIZE Force file size to this. Default is 2*RAM size. This is unlikely to be needed, unless in strange diskspace situations. Note that swap enabled and smaller than RAM causes kernal-internal VM trouble on random systems. CONF_SWAPFACTOR Set the relation between RAM and swap size. Must be an integer. Defaults to 2 which means swap size = 2 * RAM size CONF_MAXSWAP Set maximum size of the swap file in MBytes. Defaults to 2048 which was the former kernel limit for the swapfile size and is now a limit to prevent unusual big swap files on systems with a lot of RAM. FILES
/etc/dphys-swapfile user config $CONF_SWAPFILE the swap file, target of the whole action (defaults to /var/swap) EXAMPLES
dphys-swapfile is usually run at system startup and shutdown from an /etc/init.d (or /etc/rc.d) script, such as this (minimal) one: #!/bin/sh # /etc/init.d/dphys-swapfile - automatically set up an swapfile # author franklin, last modification 2004.06.04 # This script is copyright ETH Zuerich Physics Departement, # use under either modified/non-advertising BSD or GPL license case "$1" in start) /sbin/dphys-swapfile setup /sbin/dphys-swapfile swapon ;; stop) /sbin/dphys-swapfile swapoff ;; esac exit 0 If an sysadmin wants to have his swapfile in annother place, say /var/run/swap, he can use: In /etc/dphys-swapfile: CONF_SWAPFILE=/var/run/swap AUTHOR
franklin@phys.ethz.ch, http://www.phys.ethz.ch/~franklin/ D-PHYS Swapfile Tools 2006.09.15 DPHYS-SWAPFILE(8)
Check Out this Related Man Page
SWAPON(8) Linux Programmer's Manual SWAPON(8) NAME
swapon, swapoff - enable/disable devices and files for paging and swapping SYNOPSIS
Get info: swapon -s [-h] [-V] Enable/disable: swapon [-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. -e, --ifexists Silently skip devices that do not exist. -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-ng package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/. Linux 1.x 25 September 1995 SWAPON(8)