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Top Forums UNIX for Dummies Questions & Answers Next Question: Post 17481 by chenly on Friday 15th of March 2002 05:49:34 PM
Old 03-15-2002
Lightbulb 4 out of 5 UNIX users prefer dedicated swap RAIDs. Got swap?

Swap files (in UNIX, 76.2 megabytes per) are essentially virtual memory; that is, hard disk space used as RAM. Swap is much more complex than this, so suffice it to say: dedicated swap RAIDs are best; followed by dedicated swap disks; then dedicated swap partitions; and finally, nondedicated swap directories. You want the fastest, least fragmented hard disk space available for your swap files.
Test Your Knowledge in Computers #1000
Difficulty: Medium
Trusted Computer System Evaluation Criteria (TCSEC) is frequently referred to as the Red Book.
True or False?

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DPHYS-SWAPFILE(8)					      System Manager's Manual						 DPHYS-SWAPFILE(8)

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, D-PHYS Swapfile Tools 2006.09.15 DPHYS-SWAPFILE(8)

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