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DM(4)			 BSD Kernel Interfaces Manual			 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 configura‐
     tion 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 begin‐
     ning 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-map‐
     per 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

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

SEE ALSO
     config(1), proplib(3), MAKEDEV(8), dmsetup(8), fsck(8), lvm(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 work-in-progress—there can be bugs.

BSD				August 30, 2008				   BSD

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