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.

-q, --quiet

Suppress output and warning messages.

-L, --label label

Specify a label for the device, to allow swapon(8) 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 systemd-udevd(8) 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. The format of the UUID is a series of hex digits separated by hyphens, like this: "c1b9d5a2-f162-11cf-9ece-0020afc76f16". The UUID parameter may also be one of the following:

clear

clear the filesystem UUID

random

generate a new randomly-generated UUID

time

generate a new time-based UUID

-e, --endianness ENDIANNESS

Specify the ENDIANNESS to use, valid arguments are native, little or big. The default is native.

-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).)

--verbose

Verbose execution. With this option mkswap will output more details about detected problems during swap area set up.

-h, --help

Display help text and exit.

-V, --version

Print version and exit.

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.

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 can look it up with getconf PAGESIZE.

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).

For bug reports, use the issue tracker at <https://github.com/util-linux/util-linux/issues>.

The mkswap command is part of the util-linux package which can be downloaded from Linux Kernel Archive <https://www.kernel.org/pub/linux/utils/util-linux/>.