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 parti­
       tions.  (Warning: Solaris also uses this type. Be  careful
       not to kill your Solaris partitions.)

       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.  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

       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 tak­
       ing 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  archi­
       tecture 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 prob­
              ably 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.

       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)


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