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 notto 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
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
mkswap, like many others mkfs-like utils, erases the first partitionblock to make any previous filesystem invisible.
However, mkswap refuses to erase the first block on a device with a
disk label (SUN, BSD, ...).
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.
Go ahead even if the command is stupid. This allows the
creation of a swap area larger than the file or partition it
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).)
Display help text and exit.
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 (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/swapsmkswap 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.
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
firstname.lastname@example.org. This page was obtained from the
project's upstream Git repository
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 email@example.com
util-linux March 2009 MKSWAP(8)