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

NAME
       mkswap - set up a Linux swap area

SYNOPSIS
       mkswap [-c] [-vN] [-f] 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 parti-
       tions.

       The size parameter is superfluous but retained  for  back-
       wards  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.)

       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 MB if you believe in 1 MB=2^20 bytes), and the
       rest is wasted.  On an alpha and sparc64, with S=8192, the
       useful area is at most 535560992 bytes (almost 512 MB with
       the same proviso).

       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  2GB
       on  i386,  PPC,  m68k,  ARM,  1GB on sparc, 512MB on mips,
       128GB on alpha and 3TB on sparc64.

       Note that before 2.1.117 the kernel allocated one byte for
       each  page, while it now allocates two bytes, so that tak-
       ing a swap area of 2 GB in use might require 2 MB of  ker-
       nel 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  archi-
       tecture and kernel version).

       To  setup a swap file, it is necessary to create that file
       before running 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     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.

       -v0    Create an old style swap area.

       -v1    Create a new style swap area.

       If no -v option is given, mkswap will default to old style
       if the size of the swap area does not exceed  the  maximum
       size  of  an  old style swap area, and also 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 (also if the  swap  area
       is  small),  in  case you have a boot loader or disk label
       there.

SEE ALSO
       fdisk(8), swapon(8)

Linux 2.2.4               25 March 1998                         1