Provided by: util-linux_2.13.1-5ubuntu1_i386 bug

NAME

       mkswap - set up a Linux swap area

SYNOPSIS

       mkswap [-c] [-vN] [-f] [-p PSZ] [-L label] [-U uuid] 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
       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 depends on the architecture  and
       the  kernel  version.  It is roughly 2GiB on i386, PPC, m68k, 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 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 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

       -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.

       -L label
              Specify  a label, to allow swapon by label.  (Only for new style
              swap areas.)

       -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.

       Version  0  (-v0)  swap  space  format  is  no longer supported in 2.5+
       kernels.

       -U uuid
              Specify the uuid to use.

SEE ALSO

       fdisk(8), swapon(8)

AVAILABILITY

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