mkswap

Setup a given partition as swap area

>_ sudo mkswap [/dev/sdb7]

Use a given file as swap area

>_ sudo mkswap [path/to/file]

Check a partition for bad blocks before creating the swap area

>_ sudo mkswap -c [/dev/sdb7]

Specify a label for the file (to allow swapon to use the label)

>_ sudo mkswap -L [swap1] [path/to/file]

mkswap [options] device [size]

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.

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, ... ).

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

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

-U,--uuidUUID 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.

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).

LIBBLKID_DEBUG=all enables libblkid debug output.

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

fdisk(8), swapon(8)