.\"	$NetBSD: dm.4,v 1.1 2008/12/22 02:28:28 haad Exp $
.\"
.\" Copyright (c) 1996, 1997 The NetBSD Foundation, Inc.
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Adam Hamsik
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.Dd August 30, 2008
.Dt DM 4
.Os
.Sh NAME
.Nm dm
.Nd Device-mapper disk driver
.Sh SYNOPSIS
.Cd "pseudo-device dm"
.Sh DESCRIPTION
The
.Nm
driver provides the capability of creating one or more virtual disks
based on target mapping.
.Pp
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 linux lvm2tools to create/manage
lvm in NetBSD. 
.Pp
Note that there are 
.Pa linear, zero, error 
targets implemented now.
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 the
.Nm Ns 's
disklabel.
In i386 it is offset 65s. Where 63 sectors are initial boot 
sectors and 2 sectors are used for disklabel which is set to 
be read-only.
.Pp
In order to compile in support for the
.Nm ,
you must add a line similar
to the following to your kernel configuration file:
.Bd -unfilled -offset indent
pseudo-device	dm	 #device-mapper disk device
.Ed
.Pp
A
.Nm
may create linear mapped devices, zero and error block devices. Zero 
and error block devices are used mostly for testing. 
.Pa Linear 
devices are used to create Virtual disks with lineary mapped virtual blocks
to blocks on real disk. 
.Nm
Device-mapper devices are controlled throught /dev/mapper/control device. For 
controlling this device Ioctl calls are used. For implementation of communication 
channel proplib library is used. Protocol channel is defined as on proplib 
dictionary with needed values. For more reference look at sys/dev/dm/netbsd-dm.h.
.Nm
Before any device can be used every device-mapper disk device must be initialized. 
For initialization is needed to pass one line to kernel driver in a form of proplib
dictionary. Every device can have more than one tables active.
.Bd -unfilled -offset indent
0 10240 linear /dev/wd1a 384
.Ed
.Pp
.Nm
The first parameter is start sector for table defined with this line, the second is length in 
sectors which is described with this table. The third parameter is target name. All other parts
of this line depends on choosen target. 
.Nm
For linear target there are two additional parameters the first parameter describes disk device
on which device-mapper disk is mapped. The second parameter is offset on this disk from start of 
disk/partition.
.Sh WARNINGS
 This driver is still work-in-progress there can be bugs.
.Sh AUTHORS
.An Adam Hamsik Aq haad@NetBSD.org
implemented the device-mapper driver
.Nx .
.Pp
.An Brett Blymn Aq blymn@NetBSD.org , 
.An Reinoud Zandijk Aq reinoud@netbsd.org and 
.An Bill Stouder-Studenmund Aq wrstuden@netbsd.org 
provided guidance and answered questions about NetBSD implementation.
.Sh SEE ALSO
.Xr config 1 ,
.Xr MAKEDEV 8 ,
.Xr lvm 8 ,
.Xr dmsetup 8 ,
.Xr fsck 8 ,
.Xr mount 8 ,
.Xr newfs 8 ,
.Xr proplib 3
.Sh HISTORY
The device-mapper disk driver first appeared in NetBSD 6.0.