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RedHat 9 (Linux i386) - man page for mkswap (redhat section 8)

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

       mkswap - set up a Linux swap area

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

       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 script.)

       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  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.

       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

       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  allo-
       cates  two bytes, so that taking a swap area of 2 GiB in use might require 2 MiB of kernel

       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  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).

       -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  cur-
       rent  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.

       fdisk(8), swapon(8)

Linux 2.2.4				  25 March 1999 				MKSWAP(8)

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