mkswap(8) - phpMan

MKSWAP(8) System Administration MKSWAP(8)

NAME
mkswap - set up a Linux swap area

SYNOPSIS
mkswap [options] device [size]

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 par-
titions. (Warning: Solaris also uses this type. Be careful not to kill your Solaris par-
titions.)

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 parti-
tion 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 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, --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.

-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).)

-h, --help
Display help text and exit.

-V, --version
Display version information and exit.

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 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

# 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, prealloca-
tion and copy-on-write issues).

ENVIRONMENT
LIBBLKID_DEBUG=all
enables libblkid debug output.