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 \fB\-\-lock\fR[=\fImode\fR]
  80 Use exclusive BSD lock for device or file it operates.  The optional argument
  81 \fImode\fP can be \fByes\fR, \fBno\fR (or 1 and 0) or \fBnonblock\fR.  If the \fImode\fR
  82 argument is omitted, it defaults to \fB"yes"\fR.  This option overwrites
  83 environment variable \fB$LOCK_BLOCK_DEVICE\fR.  The default is not to use any
  84 lock at all, but it's recommended to avoid collisions with udevd or other
  85 tools.
  86 .TP
  87 .BR \-p , " \-\-pagesize " \fIsize\fR
  88 Specify the page \fIsize\fR (in bytes) to use.  This option is usually unnecessary;
  89 .B mkswap
  90 reads the size from the kernel.
  91 .TP
  92 .BR \-U , " \-\-uuid " \fIUUID\fR
  93 Specify the \fIUUID\fR to use.  The default is to generate a UUID.
  94 .TP
  95 .BR \-v , " \-\-swapversion 1"
  96 Specify the swap-space version.  (This option is currently pointless, as the old
  97 .B \-v 0
  98 option has become obsolete and now only
  99 .B \-v 1
 100 is supported.
 101 The kernel has not supported v0 swap-space format since 2.5.22 (June 2002).
 102 The new version v1 is supported since 2.1.117 (August 1998).)
 103 .TP
 104 .BR \-h , " \-\-help"
 105 Display help text and exit.
 106 .TP
 107 .BR \-V , " \-\-version"
 108 Display version information and exit.
 109
 110 .SH ENVIRONMENT
 111 .IP LIBBLKID_DEBUG=all
 112 enables libblkid debug output.
 113 .IP LOCK_BLOCK_DEVICE=<mode>
 114 use exclusive BSD lock.  The mode is "1" or "0".  See \fB\-\-lock\fR for more details.
 115
 116 .SH NOTES
 117 The maximum useful size of a swap area depends on the architecture and
 118 the kernel version.
 119
 120 The maximum number of the pages that is possible to address by swap area header
 121 is 4294967295 (32-bit unsigned int).  The remaining space on the swap device is ignored.
 122
 123 Presently, Linux allows 32 swap areas.
 124 The areas in use can be seen in the file
 125 .I /proc/swaps
 126
 127 .B mkswap
 128 refuses areas smaller than 10 pages.
 129
 130 If you don't know the page size that your machine uses, you may be
 131 able to look it up with "cat /proc/cpuinfo" (or you may not \(en
 132 the contents of this file depend on architecture and kernel version).
 133
 134 To set up a swap file, it is necessary to create that file before
 135 initializing it with
 136 .BR mkswap ,
 137 e.g.\& using a command like
 138
 139 .nf
 140 .RS
 141 # dd if=/dev/zero of=swapfile bs=1MiB count=$((8*1024))
 142 .RE
 143 .fi
 144
 145 to create 8GiB swapfile.
 146
 147 Please read notes from
 148 .BR swapon (8)
 149 about
 150 .B the swap file use restrictions
 151 (holes, preallocation and copy-on-write issues).
 152
 153 .SH SEE ALSO
 154 .BR fdisk (8),
 155 .BR swapon (8)
 156 .SH AVAILABILITY
 157 The mkswap command is part of the util-linux package and is available from
 158 https://www.kernel.org/pub/linux/utils/util-linux/.