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

NAME

       mkswap - set up a Linux swap area

SYNOPSIS

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

DESCRIPTION

       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 par-
       titions.  (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 ver-
       sions 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 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 con-
       tents 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

       -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

       fdisk(8), swapon(8)

Linux 2.2.4							   25 March 1999							 MKSWAP(8)