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NAME    [Toc]    [Back]

       mkswap - set up a Linux swap area

SYNOPSIS    [Toc]    [Back]

       mkswap [-c] [-vN] [-f] [-p PSZ] device [size]

DESCRIPTION    [Toc]    [Back]

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

       (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

       The device argument will usually be a disk  partition  (something  like
       /dev/hda4  or /dev/sdb7) but can also be a file.  The Linux kernel does
       not look at partition Id's, but many installation scripts  will	assume
       that partitions of hex type 82 (LINUX_SWAP) are meant to be swap partitions.
  (Warning: Solaris also uses this type. Be careful not  to  kill
       your Solaris partitions.)

       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 partition 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.

       Linux knows about two styles of swap areas, old style  and  new	style.
       The  last  10  bytes  of  the first page of the swap area distinguishes
       them: old style has `SWAP_SPACE', new style has `SWAPSPACE2' as	signature.

       In  the	old style, the rest of this first page was a bit map, with a 1
       bit for each usable page of the swap area.  Since the first page  holds
       this  bit  map,	the  first bit is 0.  Also, the last 10 bytes hold the
       signature. So, if the page size is  S,  an  old	style  swap  area  can
       describe  at  most 8*(S-10)-1 pages used for swapping.  With S=4096 (as
       on i386), the useful area is at most 133890048 bytes (almost 128  MiB),
       and the rest is wasted.	On an alpha and sparc64, with S=8192, the useful
 area is at most 535560992 bytes (almost 512 MiB).

       The old setup wastes most of this bitmap page, because zero bits denote
       bad blocks or blocks past the end of the swap space, and a simple integer
 suffices to indicate the size of the  swap  space,  while  the  bad
       blocks,	if any, can simply be listed. Nobody wants to use a swap space
       with hundreds of bad blocks. (I would not even use a swap space with  1
       bad block.)  In the new style swap area this is precisely what is done.
       The maximum useful size of a swap area now depends on the architecture.
       It  is  roughly	2GiB on i386, PPC, m68k, ARM, 1GiB on sparc, 512MiB on
       mips, 128GiB on alpha and 3TiB on sparc64.

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

       Presently, Linux allows 8 swap areas. 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  setup  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).

OPTIONS    [Toc]    [Back]

       -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.  On SPARC, force creation of the swap area.  Without
	      this option mkswap will refuse to create a v0 swap on  a	device
	      with  a  valid  SPARC  superblock, as that probably means one is
	      going to erase the partition table.

       -p PSZ Specify the page size to use.

       -v0    Create an old style swap area.

       -v1    Create a new style swap area.

       If no -v option is given, mkswap will default to new style, but use old
       style  if  the  current	kernel	is  older  than  2.1.117  (and also if
       PAGE_SIZE is less than 2048).  The new style header does not touch  the
       first  block,  so  may be preferable, in case you have a boot loader or
       disk label there.  If you need to use both 2.0 and 2.2 kernels, use the
       -v0 option when creating the swapspace.

SEE ALSO    [Toc]    [Back]

       fdisk(8), swapon(8)

Linux 2.2.4			 25 March 1999			     MKSWAP(8)
[ Back ]
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