Manual: mkswap

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=
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).

Manual: mkswap

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