NAME | SYNOPSIS | DESCRIPTION | WARNING | OPTIONS | NOTES | ENVIRONMENT | SEE ALSO | AVAILABILITY | COLOPHON

MKSWAP(8)                   System Administration                  MKSWAP(8)

NAME         top

       mkswap - set up a Linux swap area

SYNOPSIS         top

       mkswap [options] device [size]

DESCRIPTION         top

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

       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 partition 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         top

       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         top

       -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         top

       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         top

       LIBBLKID_DEBUG=all
              enables libblkid debug output.

SEE ALSO         top

       fdisk(8), swapon(8)

AVAILABILITY         top

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

COLOPHON         top

       This page is part of the util-linux (a random collection of Linux
       utilities) project.  Information about the project can be found at 
       ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩.  If you have a
       bug report for this manual page, send it to
       util-linux@vger.kernel.org.  This page was obtained from the
       project's upstream Git repository 
       ⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
       2016-10-04.  If you discover any rendering problems in this HTML ver‐
       sion of the page, or you believe there is a better or more up-to-date
       source for the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original man‐
       ual page), send a mail to man-pages@man7.org

util-linux                       March 2009                        MKSWAP(8)