1MKSWAP(8) System Administration MKSWAP(8)
2
6 mkswap - set up a Linux swap area
7
9 mkswap [options] device [size]
10
12 mkswap sets up a Linux swap area on a device or in a file.
13 The device argument will usually be a disk partition (something like
15 /dev/sdb7) but can also be a file. The Linux kernel does not look at
16 partition IDs, but many installation scripts will assume that parti‐
17 tions of hex type 82 (LINUX_SWAP) are meant to be swap partitions.
18 (Warning: Solaris also uses this type. Be careful not to kill your
19 Solaris partitions.)
20 The size parameter is superfluous but retained for backwards compati‐
22 bility. (It specifies the desired size of the swap area in 1024-byte
23 blocks. mkswap will use the entire partition or file if it is omitted.
24 Specifying it is unwise – a typo may destroy your disk.)
25 After creating the swap area, you need the swapon command to start
27 using it. Usually swap areas are listed in /etc/fstab so that they can
28 be taken into use at boot time by a swapon -a command in some boot
29 script.
30
33 The swap header does not touch the first block. A boot loader or disk
34 label can be there, but it is not a recommended setup. The recommended
35 setup is to use a separate partition for a Linux swap area.
36 mkswap, like many others mkfs-like utils, erases the first partition
38 block to make any previous filesystem invisible.
39 However, mkswap refuses to erase the first block on a device with a
41 disk label (SUN, BSD, ...).
42
45 -c, --check
46 Check the device (if it is a block device) for bad blocks before
47 creating the swap area. If any bad blocks are found, the count
48 is printed.
49 -f, --force
51 Go ahead even if the command is stupid. This allows the cre‐
52 ation of a swap area larger than the file or partition it
53 resides on.
54 Also, without this option, mkswap will refuse to erase the first
56 block on a device with a partition table.
57 -L, --label label
59 Specify a label for the device, to allow swapon by label.
60 -p, --pagesize size
62 Specify the page size (in bytes) to use. This option is usually
63 unnecessary; mkswap reads the size from the kernel.
64 -U, --uuid UUID
66 Specify the UUID to use. The default is to generate a UUID.
67 -v, --swapversion 1
69 Specify the swap-space version. (This option is currently
70 pointless, as the old -v 0 option has become obsolete and now
71 only -v 1 is supported. The kernel has not supported v0 swap-
72 space format since 2.5.22 (June 2002). The new version v1 is
73 supported since 2.1.117 (August 1998).)
74 -h, --help
76 Display help text and exit.
77 -V, --version
79 Display version information and exit.
80
83 The maximum useful size of a swap area depends on the architecture and
84 the kernel version.
85 The maximum number of the pages that is possible to address by swap
87 area header is 4294967295 (UINT_MAX). The remaining space on the swap
88 device is ignored.
89 Presently, Linux allows 32 swap areas. The areas in use can be seen in
91 the file /proc/swaps
92 mkswap refuses areas smaller than 10 pages.
94 If you don't know the page size that your machine uses, you may be able
96 to look it up with "cat /proc/cpuinfo" (or you may not – the contents
97 of this file depend on architecture and kernel version).
98 To set up a swap file, it is necessary to create that file before ini‐
100 tializing it with mkswap, e.g. using a command like
101 # fallocate --length 8GiB swapfile
103 Note that a swap file must not contain any holes. Using cp(1) to cre‐
105 ate the file is not acceptable. Neither is use of fallocate(1) on file
106 systems that support preallocated files, such as XFS or ext4, or on
107 copy-on-write filesystems like btrfs. It is recommended to use dd(1)
108 and /dev/zero in these cases. Please read notes from swapon(8) before
109 adding a swap file to copy-on-write filesystems.
110
113 LIBBLKID_DEBUG=all
114 enables libblkid debug output.
115
118 fdisk(8), swapon(8)
119
121 The mkswap command is part of the util-linux package and is available
122 from https://www.kernel.org/pub/linux/utils/util-linux/.
123util-linux March 2009 MKSWAP(8)