set up a Linux swap area
[options] device [size]
sets up a Linux swap area on a device or in a file.
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.)
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.)
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.
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
like many others mkfs-like utils, erases the first
partition block to make any previous filesystem
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 resides on.
this option, mkswap will refuse to erase the first
block on a device with a partition table.
Specify a label for the
device, to allow swapon by label.
Specify the page size
(in bytes) to use. This option is usually unnecessary;
mkswap reads the size from the kernel.
Specify the UUID to use.
The default is to generate a UUID.
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
useful size of a swap area depends on the architecture and
the kernel version.
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.
Linux allows 32 swap areas. The areas in use can be seen in
the file /proc/swaps
refuses areas smaller than 10 pages.
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
−−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.
enables libblkid debug
command is part of the util-linux package and is available