DM(4)

NAME
dm - Device-mapper disk driver

SYNOPSIS
pseudo-device dm

DESCRIPTION
The dm driver provides the capability of creating one or more virtual disks based on the target mapping.

This document assumes that you're familiar with how to generate kernels, how to properly configure disks and pseudo-devices in a kernel configuration file, and how to partition disks. This driver is used by the Linux lvm2tools to create and manage lvm in NetBSD.

Currently, the linear, zero, and error targets are implemented. Each component partition should be offset at least 2 sectors from the beginning of the component disk. This avoids potential conflicts between the component disk's disklabel and dm's disklabel. In i386 it is offset by 65 sectors, where 63 sectors are the initial boot sectors and 2 sectors are used for the disklabel which is set to be read-only.

In order to compile in support for dm, you must add a line similar to the following to your kernel configuration file:

    pseudo-device	dm	 #device-mapper disk device

dm may create linear mapped devices, zero, and error block devices. Zero and error block devices are used mostly for testing. Linear devices are used to create virtual disks with linearly mapped virtual blocks to blocks on real disk. dm Device-mapper devices are controlled through the /dev/mapper/control device. For controlling this device ioctl(2) calls are used. For the implementation of the communication channel, the proplib(3) library is used. The protocol channel is defined as a proplib dictionary with needed values. For more details, look at sys/dev/dm/netbsd-dm.h. Before any device can be used, every device-mapper disk device must be initialized. For initialization one line must be passed to the kernel driver in the form of a proplib dictionary. Every device can have more than one table active. An example for such a line is:

    0 10240 linear /dev/wd1a 384

The first parameter is the start sector for the table defined with this line, the second is the length in sectors which is described with this table. The third parameter is the target name. All other parts of this line depend on the chosen target.

For the linear target, there are two additional parameters: The fourth parameter describes the disk device to which the device-mapper disk is mapped. The fifth parameter is the offset on this disk from the start of the disk/partition.

SEE ALSO
config(1), proplib(3), dmsetup(8), fsck(8), lvm(8), MAKEDEV(8), mount(8), newfs(8)

HISTORY
The device-mapper disk driver first appeared in NetBSD 6.0.

AUTHORS
Adam Hamsik <haad@NetBSD.org> implemented the device-mapper driver for NetBSD.

Brett Lymn <blymn@NetBSD.org>, Reinoud Zandijk <reinoud@NetBSD.org>, and Bill Stouder-Studenmund <wrstuden@NetBSD.org> provided guidance and answered questions about the NetBSD implementation.

BUGS
This driver is still a work-in-progress — there can be bugs.