mkswap

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 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
       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.

       --lock[=mode]
              Use exclusive BSD lock for device or file it operates.  The
              optional argument mode can be yes, no (or 1 and 0) or nonblock.
              If the mode argument is omitted, it defaults to "yes".  This
              option overwrites environment variable $LOCK_BLOCK_DEVICE.  The
              default is not to use any lock at all, but it's recommended to
              avoid collisions with udevd or other tools.

       -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.


ENVIRONMENT
       LIBBLKID_DEBUG=all
              enables libblkid debug output.

       LOCK_BLOCK_DEVICE=<mode>
              use exclusive BSD lock.  The mode is "1" or "0".  See --lock for
              more details.


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 (32-bit unsigned int).  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

              # dd if=/dev/zero of=swapfile bs=1MiB count=$((8*1024))

       to create 8GiB swapfile.

       Please read notes from swapon(8) about the swap file use restrictions
       (holes, preallocation and copy-on-write issues).


SEE ALSO
       fdisk(8), swapon(8)

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



util-linux                        March 2009                         MKSWAP(8)