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       --lock[=mode]
62              Use  exclusive  BSD  lock  for  device or file it operates.  The
63              optional argument mode can be yes, no (or 1 and 0) or  nonblock.
64              If  the  mode  argument  is omitted, it defaults to "yes".  This
65              option overwrites environment variable $LOCK_BLOCK_DEVICE.   The
66              default  is  not to use any lock at all, but it's recommended to
67              avoid collisions with udevd or other tools.
68
69       -p, --pagesize size
70              Specify the page size (in bytes) to use.  This option is usually
71              unnecessary; mkswap reads the size from the kernel.
72
73       -U, --uuid UUID
74              Specify the UUID to use.  The default is to generate a UUID.
75
76       -v, --swapversion 1
77              Specify  the  swap-space  version.   (This  option  is currently
78              pointless, as the old -v 0 option has become  obsolete  and  now
79              only  -v  1 is supported.  The kernel has not supported v0 swap-
80              space format since 2.5.22 (June 2002).  The new  version  v1  is
81              supported since 2.1.117 (August 1998).)
82
83       -h, --help
84              Display help text and exit.
85
86       -V, --version
87              Display version information and exit.
88
89

ENVIRONMENT

91       LIBBLKID_DEBUG=all
92              enables libblkid debug output.
93
94       LOCK_BLOCK_DEVICE=<mode>
95              use exclusive BSD lock.  The mode is "1" or "0".  See --lock for
96              more details.
97
98

NOTES

100       The maximum useful size of a swap area depends on the architecture  and
101       the kernel version.
102
103       The  maximum  number  of  the pages that is possible to address by swap
104       area header is 4294967295 (32-bit unsigned int).  The  remaining  space
105       on the swap device is ignored.
106
107       Presently, Linux allows 32 swap areas.  The areas in use can be seen in
108       the file /proc/swaps
109
110       mkswap refuses areas smaller than 10 pages.
111
112       If you don't know the page size that your machine uses, you may be able
113       to  look  it up with "cat /proc/cpuinfo" (or you may not – the contents
114       of this file depend on architecture and kernel version).
115
116       To set up a swap file, it is necessary to create that file before  ini‐
117       tializing it with mkswap, e.g. using a command like
118
119              # dd if=/dev/zero of=swapfile bs=1MiB count=$((8*1024))
120
121       to create 8GiB swapfile.
122
123       Please  read  notes from swapon(8) about the swap file use restrictions
124       (holes, preallocation and copy-on-write issues).
125
126

AVAILABILITY

131       The mkswap command is part of the util-linux package and  is  available
132       from https://www.kernel.org/pub/linux/utils/util-linux/.
133
134
135
136util-linux                        March 2009                         MKSWAP(8)