|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | SYNOPSIS | DESCRIPTION | WARNING | OPTIONS | ENVIRONMENT | NOTES | SEE ALSO | REPORTING BUGS | AVAILABILITY |
|
|
|
MKSWAP(8) System Administration MKSWAP(8)
mkswap - set up a Linux swap area
mkswap [options] device [blocks]
mkswap [options] --size size --file file
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 blocks 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(8) 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, --file
Create a swap file with the appropriate file permissions and
populated blocks on disk.
-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 an exclusive BSD lock for the device or file that is
operated upon. The optional argument mode can be yes (1), no
(0), or nonblock. If the mode argument is omitted, it defaults
to yes. This option overrides the environment variable
$LOCK_BLOCK_DEVICE. The default is to not use any lock at all,
but using a lock is 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.
-o, --offset offset
Specify the offset to write the swap area to.
-s, --size size
Specify the size of the created swap file in bytes and may be
followed by a multiplicative suffix: KiB (=1024), MiB
(=1024*1024), and so on for GiB, TiB, PiB, EiB, ZiB and YiB
(the "iB" is optional, e.g., "K" has the same meaning as
"KiB"). If the file exists and is larger than size, it will be
truncated to this size. This option only makes sense when used
with --file.
-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
Display 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.
Aside from mkswap --file, it is also possible to create the
swapfile manually before initializing it with mkswap, e.g. using a
command like
Since version 2.41, mkswap --file sets the nocow attribute for
newly created files to support swapfiles on Btrfs.
# dd if=/dev/zero of=swapfile bs=1MiB count=$((8*1024))
to create 8GiB swapfile.
In such a case, please read notes from swapon(8) about the swap
file use restrictions (holes, preallocation and copy-on-write
issues).
fdisk(8), swapon(8)
For bug reports, use the issue tracker
<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/>. 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
[email protected]. This page was obtained from the
project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
2025-08-11. (At that time, the date of the most recent commit that
was found in the repository was 2025-08-05.) If you discover any
rendering problems in this HTML version 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 manual page), send a
mail to [email protected]
util-linux 2.42-start-521-ec46 2025-08-09 MKSWAP(8)
Pages that refer to this page: swapon(2), crypttab(5), swaplabel(8), swapon(8)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|