mkswap — set up a Linux swap area
mkswap [−c] [−f] [ −p PSZ ] [ −L label ] [ −U uuid ] device [size]
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 Id's, 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 killyour 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.)
The PSZ parameter
specifies the page size to use. It is almost always
unnecessary (even unwise) to specify it, but certain old libc
versions lie about the page size, so it is possible that
mkswap gets it
wrong. The symptom is that a subsequent swapon fails because no
swap signature is found. Typical values for PSZ are 4096 or 8192.
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.
The swap header does not touch the first block. A boot loader or disk label can be there, but it is not 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 block to remove old on-disk filesystems.
mkswap refuses to erase the first block on a device with a disk label (SUN, BSD, ...) or on whole disk (e.g. /dev/sda).
−cCheck the device (if it is a block device) for bad blocks before creating the swap area. If any are found, the count is printed.
−fForce - 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.
Without this option mkswap will refuse to erase the first block on a device with a partition table or on whole disk (e.g. /dec/sda).
−p
PSZSpecify the page size to use.
−L
labelSpecify a label, to allow swapon by label. (Only for new style swap areas.)
−v0,
−v1Specify the swap space version. This option is deprecated and −v1 is supported only.
The kernel has not supported v0 swap space format since 2.5.22. The new version v1 is supported since 2.1.117.
−U
uuidSpecify the uuid to use. The default is to generate UUIDs.
The maximum useful size of a swap area depends on the architecture and the kernel version. It is roughly 2GiB on i386, PPC, m68k, ARM, 1GiB on sparc, 512MiB on mips, 128GiB on alpha and 3TiB on sparc64. For kernels after 2.3.3 there is no such limitation.
Note that before 2.1.117 the kernel allocated one byte for each page, while it now allocates two bytes, so that taking a swap area of 2 GiB in use might require 2 MiB of kernel memory.
Presently, Linux allows 32 swap areas (this was 8 before
Linux 2.4.10). The areas in use can be seen in the file
/proc/swaps (since 2.1.25).
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 setup 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).