1MKSWAP(8)                  Linux Programmer's Manual                 MKSWAP(8)
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NAME

6       mkswap - set up a Linux swap area
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SYNOPSIS

9       mkswap [-c] [-f] [-p PSZ] [-L label] [-U uuid] device [size]
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DESCRIPTION

12       mkswap sets up a Linux swap area on a device or in a file.
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14       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.)
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21       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.)
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26       The  PSZ parameter specifies the page size to use.  It is almost always
27       unnecessary (even unwise) to specify it, but certain old libc  versions
28       lie  about  the page size, so it is possible that mkswap gets it wrong.
29       The symptom is that a subsequent swapon fails because no swap signature
30       is found.  Typical values for PSZ are 4096 or 8192.
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32       After  creating  the  swap  area,  you need the swapon command to start
33       using it.  Usually swap areas are listed in /etc/fstab so that they can
34       be  taken  into  use  at  boot time by a swapon -a command in some boot
35       script.
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WARNING

39       The swap header does not touch the first block.  A boot loader or  disk
40       label can be there, but it is not a recommended setup.  The recommended
41       setup is to use a separate partition for a Linux swap area.
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43       mkswap, like many others mkfs-like utils, erases  the  first  block  to
44       remove old on-disk filesystems.
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46       mkswap  refuses  to erase the first block on a device with a disk label
47       (SUN, BSD, ...) or on a whole disk (e.g. /dev/sda).
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OPTIONS

51       -c     Check the device (if it is a block device) for bad blocks before
52              creating the swap area.  If any are found, the count is printed.
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54       -f     Force  --  go  ahead even if the command is stupid.  This allows
55              the creation of a swap area larger than the file or partition it
56              resides on.
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58              Without this option, mkswap will refuse to erase the first block
59              on a device with a partition table or  on  a  whole  disk  (e.g.
60              /dev/sda).
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62       -L label
63              Specify a label, to allow swapon by label.
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65       -p PSZ Specify the page size (in bytes) to use.  This option is usually
66              unnecessary, mkswap reads the size from the kernel.
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68       -U uuid
69              Specify the uuid to use.  The default is to generate a UUID.
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71       -v1    Specify the swap-space version.  The old -v0 option  has  become
72              obsolete and now only -v1 is supported.
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74              The  kernel has not supported v0 swap-space format since 2.5.22.
75              The new version v1 is supported since 2.1.117.
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NOTES

79       The maximum useful size of a swap area depends on the architecture  and
80       the  kernel  version.   It  is roughly 2GiB on i386, PPC, m68k and ARM,
81       1GiB on sparc, 512MiB on mips, 128GiB on alpha, and  3TiB  on  sparc64.
82       For kernels after 2.3.3 there is no such limitation.
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84       Note that before version 2.1.117 the kernel allocated one byte for each
85       page, while it now allocates two bytes, so that taking into use a  swap
86       area of 2 GiB might require 2 MiB of kernel memory.
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88       Presently, Linux allows 32 swap areas (this was 8 before Linux 2.4.10).
89       The areas in use can be seen in the file /proc/swaps (since 2.1.25).
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91       mkswap refuses areas smaller than 10 pages.
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93       If you don't know the page size that your machine uses, you may be able
94       to  look it up with "cat /proc/cpuinfo" (or you may not -- the contents
95       of this file depend on architecture and kernel version).
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97       To set up a swap file, it is necessary to create that file before  ini‐
98       tializing it with mkswap, e.g. using a command like
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100              # dd if=/dev/zero of=swapfile bs=1024 count=65536
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102       Note  that  a  swap file must not contain any holes (so, using cp(1) to
103       create the file is not acceptable).
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SEE ALSO

108       fdisk(8), swapon(8)
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AVAILABILITY

111       The mkswap command is part of the util-linux-ng package and  is  avail‐
112       able from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.
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116Linux                            13 March 2009                       MKSWAP(8)