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MKSWAP(8)                             System Administration                             MKSWAP(8)

NAME
       mkswap - set up a Linux swap area

SYNOPSIS
       mkswap [options] 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 IDs,  but  many  installation
       scripts  will assume that partitions of hex type 82 (LINUX_SWAP) are meant to be swap par‐
       titions.  (Warning: Solaris also uses this type.  Be careful not to kill your Solaris par‐
       titions.)

       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  parti‐
       tion or file if it is omitted.  Specifying it is unwise – a typo may destroy your disk.)

       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 a 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 partition block to make any
       previous filesystem invisible.

       However, mkswap refuses to erase the first block on a device with a disk label (SUN,  BSD,
       ...).

OPTIONS
       -c, --check
              Check  the device (if it is a block device) for bad blocks before creating the swap
              area.  If any bad blocks 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.

              Also,  without this option, mkswap will refuse to erase the first block on a device
              with a partition table.

       -L, --label label
              Specify a label for the device, to allow swapon by label.

       -p, --pagesize size
              Specify the page size (in bytes) to  use.   This  option  is  usually  unnecessary;
              mkswap reads the size from the kernel.

       -U, --uuid UUID
              Specify the UUID to use.  The default is to generate a UUID.

       -v, --swapversion 1
              Specify the swap-space version.  (This option is currently pointless, as the old -v
              0 option has become obsolete and now only -v 1 is supported.  The  kernel  has  not
              supported  v0  swap-space  format  since 2.5.22 (June 2002).  The new version v1 is
              supported since 2.1.117 (August 1998).)

       -h, --help
              Display help text and exit.

       -V, --version
              Display version information and exit.

NOTES
       The maximum useful size of a swap area depends on the architecture and the kernel version.

       The maximum number of the pages that is  possible  to  address  by  swap  area  header  is
       4294967295 (UINT_MAX).  The remaining space on the swap device is ignored.

       Presently,  Linux  allows  32  swap  areas.   The  areas  in  use  can be seen in the file
       /proc/swaps

       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 set up a swap file, it is necessary to create that file  before  initializing  it  with
       mkswap, e.g. using a command like

              # fallocate --length 8GiB swapfile

       Note  that  a swap file must not contain any holes.  Using cp(1) to create the file is not
       acceptable.  Neither is use of fallocate(1) on  file  systems  that  support  preallocated
       files, such as XFS or ext4, or on copy-on-write filesystems like btrfs.  It is recommended
       to use dd(1) and /dev/zero in these cases.  Please read notes from swapon(8) before adding
       a swap file to copy-on-write filesystems.

ENVIRONMENT
       LIBBLKID_DEBUG=all
              enables libblkid debug output.

SEE ALSO
       fdisk(8), swapon(8)

AVAILABILITY
       The  mkswap command is part of the util-linux package and is available from ftp://ftp.ker‐
       nel.org/pub/linux/utils/util-linux/.

util-linux                                  March 2009                                  MKSWAP(8)

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