1MKSWAP(8) Linux Programmer's Manual MKSWAP(8)
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6 mkswap - set up a Linux swap area
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9 mkswap [-c] [-vN] [-f] [-p PSZ] [-L label] device [size]
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12 mkswap sets up a Linux swap area on a device or in a file.
13 (After creating the swap area, you need the swapon command to start
15 using it. Usually swap areas are listed in /etc/fstab so that they can
16 be taken into use at boot time by a swapon -a command in some boot
17 script.)
18 The device argument will usually be a disk partition (something like
20 /dev/hda4 or /dev/sdb7) but can also be a file. The Linux kernel does
21 not look at partition Id's, but many installation scripts will assume
22 that partitions of hex type 82 (LINUX_SWAP) are meant to be swap parti‐
23 tions. (Warning: Solaris also uses this type. Be careful not to kill
24 your Solaris partitions.)
25 The size parameter is superfluous but retained for backwards compati‐
27 bility. (It specifies the desired size of the swap area in 1024-byte
28 blocks. mkswap will use the entire partition or file if it is omitted.
29 Specifying it is unwise - a typo may destroy your disk.)
30 The PSZ parameter specifies the page size to use. It is almost always
32 unnecessary (even unwise) to specify it, but certain old libc versions
33 lie about the page size, so it is possible that mkswap gets it wrong.
34 The symptom is that a subsequent swapon fails because no swap signature
35 is found. Typical values for PSZ are 4096 or 8192.
36 Linux knows about two styles of swap areas, old style and new style.
38 The last 10 bytes of the first page of the swap area distinguishes
39 them: old style has `SWAP_SPACE', new style has `SWAPSPACE2' as signa‐
40 ture.
41 In the old style, the rest of this first page was a bit map, with a 1
43 bit for each usable page of the swap area. Since the first page holds
44 this bit map, the first bit is 0. Also, the last 10 bytes hold the
45 signature. So, if the page size is S, an old style swap area can
46 describe at most 8*(S-10)-1 pages used for swapping. With S=4096 (as
47 on i386), the useful area is at most 133890048 bytes (almost 128 MiB),
48 and the rest is wasted. On an alpha and sparc64, with S=8192, the use‐
49 ful area is at most 535560992 bytes (almost 512 MiB).
50 The old setup wastes most of this bitmap page, because zero bits denote
52 bad blocks or blocks past the end of the swap space, and a simple inte‐
53 ger suffices to indicate the size of the swap space, while the bad
54 blocks, if any, can simply be listed. Nobody wants to use a swap space
55 with hundreds of bad blocks. (I would not even use a swap space with 1
56 bad block.) In the new style swap area this is precisely what is done.
57 The maximum useful size of a swap area depends on the architecture and
59 the kernel version. It is roughly 2GiB on i386, PPC, m68k, ARM, 1GiB
60 on sparc, 512MiB on mips, 128GiB on alpha and 3TiB on sparc64. For ker‐
61 nels after 2.3.3 there is no such limitation.
62 Note that before 2.1.117 the kernel allocated one byte for each page,
64 while it now allocates two bytes, so that taking a swap area of 2 GiB
65 in use might require 2 MiB of kernel memory.
66 Presently, Linux allows 32 swap areas (this was 8 before Linux 2.4.10).
68 The areas in use can be seen in the file /proc/swaps (since 2.1.25).
69 mkswap refuses areas smaller than 10 pages.
71 If you don't know the page size that your machine uses, you may be able
73 to look it up with "cat /proc/cpuinfo" (or you may not - the contents
74 of this file depend on architecture and kernel version).
75 To setup a swap file, it is necessary to create that file before ini‐
77 tializing it with mkswap , e.g. using a command like
78 # dd if=/dev/zero of=swapfile bs=1024 count=65536
80 Note that a swap file must not contain any holes (so, using cp(1) to
82 create the file is not acceptable).
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86 -c Check the device (if it is a block device) for bad blocks before
87 creating the swap area. If any are found, the count is printed.
88 -f Force - go ahead even if the command is stupid. This allows the
90 creation of a swap area larger than the file or partition it
91 resides on. On SPARC, force creation of the swap area. Without
92 this option mkswap will refuse to create a v0 swap on a device
93 with a valid SPARC superblock, as that probably means one is
94 going to erase the partition table.
95 -p PSZ Specify the page size to use.
97 -L label
99 Specify a label, to allow swapon by label. (Only for new style
100 swap areas.)
101 -v0 Create an old style swap area.
103 -v1 Create a new style swap area.
105 If no -v option is given, mkswap will default to new style, but use old
108 style if the current kernel is older than 2.1.117 (and also if
109 PAGE_SIZE is less than 2048). The new style header does not touch the
110 first block, so may be preferable, in case you have a boot loader or
111 disk label there. If you need to use both 2.0 and 2.2 kernels, use the
112 -v0 option when creating the swapspace.
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116 fdisk(8), swapon(8)
117Linux 2.2.4 25 March 1999 MKSWAP(8)