dm-linear — The Linux Kernel 5.18.5-gnu-1 documentation

# dm-linear

Device-Mapper’s “linear” target maps a linear range of the Device-Mapper device onto a linear range of another device. This is the basic building block of logical volume managers.

Parameters: <dev path> <offset>
  <dev path>:

    Full pathname to the underlying block-device, or a “major:minor” device-number.

  <offset>:

    Starting sector within the device.

## Example scripts

    #!/bin/sh
    # Create an identity mapping for a device
    echo "0 `blockdev --getsz $1` linear $1 0" | dmsetup create identity

    #!/bin/sh
    # Join 2 devices together
    size1=`blockdev --getsz $1`
    size2=`blockdev --getsz $2`
    echo "0 $size1 linear $1 0
    $size1 $size2 linear $2 0" | dmsetup create joined

    #!/usr/bin/perl -w
    # Split a device into 4M chunks and then join them together in reverse order.

    my $name = "reverse";
    my $extent_size = 4 * 1024 * 2;
    my $dev = $ARGV[0];
    my $table = "";
    my $count = 0;

    if (!defined($dev)) {
            die("Please specify a device.\n");
    }

    my $dev_size = `blockdev --getsz $dev`;
    my $extents = int($dev_size / $extent_size) -
                  (($dev_size % $extent_size) ? 1 : 0);

    while ($extents > 0) {
            my $this_start = $count * $extent_size;
            $extents--;
            $count++;
            my $this_offset = $extents * $extent_size;

            $table .= "$this_start $extent_size linear $dev $this_offset\n";
    }

    `echo \"$table\" | dmsetup create $name`;