mkswap(8)

1MKSWAP(8)                    System Administration                   MKSWAP(8)
2
3
4

NAME

6       mkswap - set up a Linux swap area
7

SYNOPSIS

9       mkswap [options] device [size]
10

DESCRIPTION

12       mkswap sets up a Linux swap area on a device or in a file.
13
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.)
20
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.)
25
26       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
31

WARNING

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
37       mkswap, like many others mkfs-like utils, erases  the  first  partition
38       block to make any previous filesystem invisible.
39
40       However,  mkswap  refuses  to  erase the first block on a device with a
41       disk label (SUN, BSD, ...).
42
43

OPTIONS

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
50       -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
55              Also, without this option, mkswap will refuse to erase the first
56              block on a device with a partition table.
57
58       -L, --label label
59              Specify a label for the device, to allow swapon by label.
60
61       -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
65       -U, --uuid UUID
66              Specify the UUID to use.  The default is to generate a UUID.
67
68       -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
75       -h, --help
76              Display help text and exit.
77
78       -V, --version
79              Display version information and exit.
80
81

NOTES

83       The maximum useful size of a swap area depends on the architecture  and
84       the kernel version.
85
86       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
90       Presently, Linux allows 32 swap areas.  The areas in use can be seen in
91       the file /proc/swaps
92
93       mkswap refuses areas smaller than 10 pages.
94
95       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
99       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
102              # dd if=/dev/zero of=swapfile bs=1MiB count=$((8*1024))
103
104       to create 8GiB swapfile.
105
106       Note  that a swap file must not contain any holes.  Using cp(1) to cre‐
107       ate the file is not acceptable.  Neither is use of fallocate(1) on file
108       systems  that  support  preallocated  files, such as XFS or ext4, or on
109       copy-on-write filesystems like btrfs.  It is recommended to  use  dd(1)
110       and  /dev/zero in these cases.  Please read notes from swapon(8) before
111       adding a swap file to copy-on-write filesystems.
112
113

ENVIRONMENT

115       LIBBLKID_DEBUG=all
116              enables libblkid debug output.
117
118

AVAILABILITY

123       The mkswap command is part of the util-linux package and  is  available
124       from https://www.kernel.org/pub/linux/utils/util-linux/.
125
126
127
128util-linux                        March 2009                         MKSWAP(8)