MKSWAP(8) System Administration MKSWAP(8)
NAME
mkswap - set up a Linux swap area
SYNOPSIS
mkswap [options] device [blocks]
mkswap [options] --size size --file file
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 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.
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 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, ...).
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, --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 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.
-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.
ENVIRONMENT
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.
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 (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).
SEE ALSO
fdisk(8), swapon(8)
REPORTING BUGS
For bug reports, use the issue tracker
<https://github.com/util-linux/util-linux/issues>.
AVAILABILITY
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/>.
util-linux 2.41 2025-02-26 MKSWAP(8)
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