disk-utils/mkswap.8

   1 .\" Copyright 1998 Andries E. Brouwer (aeb@cwi.nl)
   2 .\"
   3 .\" May be distributed under the GNU General Public License
   4 .\"
   5 .TH MKSWAP 8 "March 2009" "util-linux" "System Administration"
   6 .SH NAME
   7 mkswap \- set up a Linux swap area
   8 .SH SYNOPSIS
   9 .B mkswap
  10 [options]
  11 .I device
  12 .RI [ size ]
  13 .SH DESCRIPTION
  14 .B mkswap
  15 sets up a Linux swap area on a device or in a file.
  16
  17 The
  18 .I device
  19 argument will usually be a disk partition (something like
  20 .IR /dev/sdb7 )
  21 but can also be a file.
  22 The Linux kernel does not look at partition IDs, but
  23 many installation scripts will assume that partitions
  24 of hex type 82 (LINUX_SWAP) are meant to be swap partitions.
  25 (\fBWarning: Solaris also uses this type.  Be careful not to kill
  26 your Solaris partitions.\fP)
  27
  28 The
  29 .I size
  30 parameter is superfluous but retained for backwards compatibility.
  31 (It specifies the desired size of the swap area in 1024-byte blocks.
  32 .B mkswap
  33 will use the entire partition or file if it is omitted.
  34 Specifying it is unwise \(en a typo may destroy your disk.)
  35
  36 After creating the swap area, you need the
  37 .B swapon
  38 command to start using it.  Usually swap areas are listed in
  39 .I /etc/fstab
  40 so that they can be taken into use at boot time by a
  41 .B swapon \-a
  42 command in some boot script.
  43
  44 .SH WARNING
  45 The swap header does not touch the first block.  A boot loader or disk label
  46 can be there, but it is not a recommended setup.  The recommended setup is to
  47 use a separate partition for a Linux swap area.
  48
  49 .BR mkswap ,
  50 like many others mkfs-like utils,
  51 .B erases the first partition block to make any previous filesystem invisible.
  52
  53 However,
  54 .B mkswap
  55 refuses to erase the first block on a device with a disk
  56 label (SUN, BSD, \&...\&).
  57
  58 .SH OPTIONS
  59 .TP
  60 .BR \-c , " \-\-check"
  61 Check the device (if it is a block device) for bad blocks
  62 before creating the swap area.
  63 If any bad blocks are found, the count is printed.
  64 .TP
  65 .BR \-f , " \-\-force"
  66 Go ahead even if the command is stupid.
  67 This allows the creation of a swap area larger than the file
  68 or partition it resides on.
  69
  70 Also, without this option,
  71 .B mkswap
  72 will refuse to erase the first block on a device with a partition table.
  73 .TP
  74 .BR \-L , " \-\-label " \fIlabel\fR
  75 Specify a \fIlabel\fR for the device, to allow
  76 .B swapon
  77 by label.
  78 .TP
  79 .BR \-p , " \-\-pagesize " \fIsize\fR
  80 Specify the page \fIsize\fR (in bytes) to use.  This option is usually unnecessary;
  81 .B mkswap
  82 reads the size from the kernel.
  83 .TP
  84 .BR \-U , " \-\-uuid " \fIUUID\fR
  85 Specify the \fIUUID\fR to use.  The default is to generate a UUID.
  86 .TP
  87 .BR \-v , " \-\-swapversion 1"
  88 Specify the swap-space version.  (This option is currently pointless, as the old
  89 .B \-v 0
  90 option has become obsolete and now only
  91 .B \-v 1
  92 is supported.
  93 The kernel has not supported v0 swap-space format since 2.5.22 (June 2002).
  94 The new version v1 is supported since 2.1.117 (August 1998).)
  95 .TP
  96 .BR \-h , " \-\-help"
  97 Display help text and exit.
  98 .TP
  99 .BR \-V , " \-\-version"
 100 Display version information and exit.
 101
 102 .SH ENVIRONMENT
 103 .IP LIBBLKID_DEBUG=all
 104 enables libblkid debug output.
 105
 106 .SH NOTES
 107 The maximum useful size of a swap area depends on the architecture and
 108 the kernel version.
 109
 110 The maximum number of the pages that is possible to address by swap area header
 111 is 4294967295 (32-bit unsigned int).  The remaining space on the swap device is ignored.
 112
 113 Presently, Linux allows 32 swap areas.
 114 The areas in use can be seen in the file
 115 .I /proc/swaps
 116
 117 .B mkswap
 118 refuses areas smaller than 10 pages.
 119
 120 If you don't know the page size that your machine uses, you may be
 121 able to look it up with "cat /proc/cpuinfo" (or you may not \(en
 122 the contents of this file depend on architecture and kernel version).
 123
 124 To set up a swap file, it is necessary to create that file before
 125 initializing it with
 126 .BR mkswap ,
 127 e.g.\& using a command like
 128
 129 .nf
 130 .RS
 131 # dd if=/dev/zero of=swapfile bs=1MiB count=$((8*1024))
 132 .RE
 133 .fi
 134
 135 to create 8GiB swapfile.
 136
 137 Please read notes from
 138 .BR swapon (8)
 139 about
 140 .B the swap file use restrictions
 141 (holes, preallocation and copy-on-write issues).
 142
 143 .SH SEE ALSO
 144 .BR fdisk (8),
 145 .BR swapon (8)
 146 .SH AVAILABILITY
 147 The mkswap command is part of the util-linux package and is available from
 148 https://www.kernel.org/pub/linux/utils/util-linux/.