linux/Documentation/device-mapper/linear.txt
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   1dm-linear
   2=========
   3
   4Device-Mapper's "linear" target maps a linear range of the Device-Mapper
   5device onto a linear range of another device.  This is the basic building
   6block of logical volume managers.
   7
   8Parameters: <dev path> <offset>
   9    <dev path>: Full pathname to the underlying block-device, or a
  10                "major:minor" device-number.
  11    <offset>: Starting sector within the device.
  12
  13
  14Example scripts
  15===============
  16[[
  17#!/bin/sh
  18# Create an identity mapping for a device
  19echo "0 `blockdev --getsize $1` linear $1 0" | dmsetup create identity
  20]]
  21
  22
  23[[
  24#!/bin/sh
  25# Join 2 devices together
  26size1=`blockdev --getsize $1`
  27size2=`blockdev --getsize $2`
  28echo "0 $size1 linear $1 0
  29$size1 $size2 linear $2 0" | dmsetup create joined
  30]]
  31
  32
  33[[
  34#!/usr/bin/perl -w
  35# Split a device into 4M chunks and then join them together in reverse order.
  36
  37my $name = "reverse";
  38my $extent_size = 4 * 1024 * 2;
  39my $dev = $ARGV[0];
  40my $table = "";
  41my $count = 0;
  42
  43if (!defined($dev)) {
  44        die("Please specify a device.\n");
  45}
  46
  47my $dev_size = `blockdev --getsize $dev`;
  48my $extents = int($dev_size / $extent_size) -
  49              (($dev_size % $extent_size) ? 1 : 0);
  50
  51while ($extents > 0) {
  52        my $this_start = $count * $extent_size;
  53        $extents--;
  54        $count++;
  55        my $this_offset = $extents * $extent_size;
  56
  57        $table .= "$this_start $extent_size linear $dev $this_offset\n";
  58}
  59
  60`echo \"$table\" | dmsetup create $name`;
  61]]
  62
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