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mkswap



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
       size  of the swap space, while the bad blocks, if any, can
       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 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.

       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)
  




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