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Linux 2.6 - man page for mkswap (linux section 8)

MKSWAP(8)			    Linux Programmer's Manual				MKSWAP(8)

NAME
       mkswap - set up a Linux swap area

SYNOPSIS
       mkswap [-c] [-f] [-p PSZ] [-L label] [-U uuid] device [size]

DESCRIPTION
       mkswap sets up a Linux swap area on a device or in a file.

       The  device  argument  will usually be a disk partition (something like /dev/sdb7) but can
       also be a file.	The Linux kernel does not look at partition IDs,  but  many  installation
       scripts	will assume that partitions of hex type 82 (LINUX_SWAP) are meant to be swap par-
       titions.  (Warning: Solaris also uses this type.  Be careful not to kill your Solaris par-
       titions.)

       The size parameter is superfluous but retained for backwards compatibility.  (It specifies
       the desired size of the swap area in 1024-byte blocks.  mkswap will use the entire  parti-
       tion or file if it is omitted.  Specifying it is unwise -- a typo may destroy your disk.)

       The  PSZ  parameter specifies the page size to use.  It is almost always unnecessary (even
       unwise) to specify it, but certain old libc versions lie about the page	size,  so  it  is
       possible that mkswap gets it wrong.  The symptom is that a subsequent swapon fails because
       no swap signature is found.  Typical values for PSZ are 4096 or 8192.

       After creating the swap area, you need the swapon command to start using it.  Usually swap
       areas are listed in /etc/fstab so that they can be taken into use at boot time by a swapon
       -a command in some boot script.

WARNING
       The swap header does not touch the first block.	A boot loader or disk label can be there,
       but  it	is not a recommended setup.  The recommended setup is to use a separate partition
       for a Linux swap area.

       mkswap, like many others mkfs-like utils, erases the first block  to  remove  old  on-disk
       filesystems.

       mkswap  refuses	to erase the first block on a device with a disk label (SUN, BSD, ...) or
       on a whole disk (e.g. /dev/sda).

OPTIONS
       -c     Check the device (if it is a block device) for bad blocks before creating the  swap
	      area.  If any are found, the count is printed.

       -f     Force  --  go  ahead  even if the command is stupid.  This allows the creation of a
	      swap area larger than the file or partition it resides on.

	      Without this option, mkswap will refuse to erase the first block on a device with a
	      partition table or on a whole disk (e.g. /dev/sda).

       -L label
	      Specify a label, to allow swapon by label.

       -p PSZ Specify  the  page  size	(in  bytes)  to use.  This option is usually unnecessary,
	      mkswap reads the size from the kernel.

       -U uuid
	      Specify the uuid to use.	The default is to generate a UUID.

       -v1    Specify the swap-space version.  The old -v0 option has  become  obsolete  and  now
	      only -v1 is supported.

	      The kernel has not supported v0 swap-space format since 2.5.22.  The new version v1
	      is supported since 2.1.117.

NOTES
       The maximum useful size of a swap area depends on the architecture and the kernel version.
       It  is  roughly	2GiB on i386, PPC, m68k and ARM, 1GiB on sparc, 512MiB on mips, 128GiB on
       alpha, and 3TiB on sparc64.  For kernels after 2.3.3 there is no such limitation.

       Note that before version 2.1.117 the kernel allocated one byte for each page, while it now
       allocates  two  bytes, so that taking into use a swap area of 2 GiB might require 2 MiB of
       kernel memory.

       Presently, Linux allows 32 swap areas (this was 8 before Linux 2.4.10).	The areas in  use
       can be seen in the file /proc/swaps (since 2.1.25).

       mkswap refuses areas smaller than 10 pages.

       If you don't know the page size that your machine uses, you may be able to look it up with
       "cat /proc/cpuinfo" (or you may not -- the contents of this file  depend  on  architecture
       and kernel version).

       To  set	up  a  swap file, it is necessary to create that file before initializing it with
       mkswap, e.g. using a command like

	      # dd if=/dev/zero of=swapfile bs=1024 count=65536

       Note that a swap file must not contain any holes (so, using cp(1) to create  the  file  is
       not acceptable).

SEE ALSO
       fdisk(8), swapon(8)

AVAILABILITY
       The  mkswap command is part of the util-linux package and is available from ftp://ftp.ker-
       nel.org/pub/linux/utils/util-linux/.

Linux					  13 March 2009 				MKSWAP(8)


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