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 recom-
       mended 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  con-
       tents 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 https://www.kernel.org/pub/linux/utils/util-linux/.

util-linux                                                          March 2009                                                           MKSWAP(8)