*BASH User Commands Ubuntu 10.04.4 LTS Server coreutils
GNU/Linux
MKSWAP(8)                  Linux Programmer's Manual                 MKSWAP(8)



NAME
       mkswap - set up a Linux swap area

SYNOPSIS
       mkswap [-c] [-f] [-p PSZ] [-L label] [-U uuid] device [size]

DESCRIPTION
       mkswap sets up a Linux swap area on a device or in a file.

       The  device  argument  will usually be a disk partition (something like
       /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 parti-
       tions 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  compati-
       bility.   (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.

       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.


WARNING
       The swap header does not touch the first block. A boot loader  or  disk
       label  can  be  there, but it is not recommended setup. The recommended
       setup is to use a separate partition for a Linux swap area.

       mkswap like many others mkfs-like  utils  erases  the  first  block  to
       remove old on-disk filesystems.

       mkswap  refuses  to erase the first block on a device with a disk label
       (SUN, BSD, ...) or on whole disk (e.g. /dev/sda).


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.

              Without  this option mkswap will refuse to erase the first block
              on a device with a  partition  table  or  on  whole  disk  (e.g.
              /dec/sda).

       -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, -v1
              Specify the swap space version. This option  is  deprecated  and
              -v1 is supported only.

              The  kernel has not supported v0 swap space format since 2.5.22.
              The new version v1 is supported since 2.1.117.


       -U uuid
              Specify the uuid to use. The default is to generate UUIDs.


NOTES
       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 ker-
       nels 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  ini-
       tializing 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).



SEE ALSO
       fdisk(8), swapon(8)

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



Linux                            13 March 2009                       MKSWAP(8)