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 partitions. (
Warning:
Solaris also uses this type. Be careful not to kill your Solaris
partitions.)
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 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 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, ...) and on a whole disk (e.g. /dev/sda).
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 and on a whole disk (e.g.
/dev/sda).
- -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. It is roughly 2GiB on i386, PPC, m68k and ARM, 1GiB on sparc,
512MiB on mips, 128GiB on alpha, and 3TiB on sparc64. For kernels after 2.3.3
(May 1999) there is no such limitation.
Note that before version 2.1.117 the kernel allocated one byte for each page,
while it now allocates two bytes, so that taking into use a swap area of 2 GiB
might require 2 MiB of kernel memory.
Presently, Linux allows 32 swap areas (this was 8 before Linux 2.4.10 (Sep
2001)). The areas in use can be seen in the file
/proc/swaps (since
2.1.25 (Sep 1997)).
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=1024 count=65536
Note that a swap file must not contain any holes (so, using
cp(1) to
create the file is not acceptable).
SEE ALSO¶
fdisk(8),
swapon(8)
AVAILABILITY¶
The mkswap command is part of the util-linux package and is available from
ftp://ftp.kernel.org/pub/linux/utils/util-linux/.