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 con-
figuration 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-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
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
Check Out this Related Man Page
chpt(8) System Manager's Manual chpt(8)
Name
chpt - change a disk partition table
Syntax
/etc/chpt [ -a ] [ -d ] [ -q ] [ -v ] [ [ -px offset size ] ... ] device
Description
The command lets you alter the partition sizes of a disk pack. Using you can tailor your system disks and their partitions to suit your
system's individual needs.
If you want to create a file system on a partition that has been modified, you must use
The standard procedure to change a partition table is:
1. Look at the current partition table using the -q option.
2. If a file system does not exist on the a partition, create one using the command.
If a file system exists on the a partition but does not contain a partition table in its superblock, copy the partition table from the
driver to the superblock using the command with the -a option.
3. Change the partition offsets and sizes using the -px option. You can change all the partitions for one pack on one command line.
The device must be either the a or c partition of the raw device, depending upon where the file system resides. For example, if the file
system resides in the a partition of an RM05 in drive 0, device is rhp0a.
A file system must exist on the a or c partition of the pack. If you do not have a file system there, create one using
Options
-a Copies the partition table in the device driver to the disk pack.
-d Copies the default partition table to the disk pack and to the current partition table in the driver. The default partition table is
the table that was built with the disk driver.
-q Runs without modifying the partition tables. This prints the partition table of the specified disk pack. It prints the default par-
tition table in the driver if there is no partition table on the disk pack.
-v Prints verbose messages showing the progress of
-px Changes the parameters of partition x on the disk pack to the specified offset and size. x is the partition you are modifying (a, b,
c, d, e, f, g, or h). Offset is the new beginning sector, and size is the new total number of sectors of the partition being modi-
fied.
Examples
This example shows how to change the partition table on an RM05 disk pack in drive 1. The commands in this example change the the size of
the h partition to include the g partition. Comments are in parenthesis to the right of commands.
% chpt -q /dev/rhp1a (view partition table)
/dev/rhp1a
No partition table found in superblock...
using default table from device driver.
Current partition table:
partition bottom top size overlap
a 0 15883 15884 c
b 16416 49855 33440 c
c 0 500383 500384 a,b,d,e,f,g,h
d 341696 357579 15884 c,g
e 358112 414047 55936 c,g
f 414048 500287 86240 c,g
g 341696 500287 158592 c,d,e,f
h 49856 341201 291346 c
%
In all of the tables generated by bottom is the offset (starting sector), top is the ending sector, and size is the number of sectors in
the partition. The overlap is the other sectors that are partially or entirely included in the partition.
% bc (basic calculator)
500287-49856 (top of g minus bottom of h)
450431
450431+1 (add 1 because it is zero-based)
450432 (size of new h partition)
%
From the query, you can see that there is no partition table in the superblock of the a partition. If this is because there is no file
system in the a partition, run the command to create one.
For this example, assume that there is a file system in the a partition of the disk, but the file system does not contain a partition table
in its superblock. Therefore, run with the -a option to copy the partition table in the driver to the superblock of the a partition.
% chpt -a /dev/rhp1a (add table to a partition)
%
Now you have a partition table to change.
% chpt -v -ph 49856 450432 /dev/rhp1a (change h)
/dev/rhp1a
New partition table:
partition bottom top size overlap
a 0 15883 15884 c
b 16416 49855 33440 c
c 0 500383 500384 a,b,d,e,f,g,h
d 341696 357579 15884 c,g,h
e 358112 414047 55936 c,g,h
f 414048 500287 86240 c,g,h
g 341696 500287 158592 c,d,e,f,h
h 49856 500287 450432 c,d,e,f,g
%
Caution
Changing partition tables indiscriminately can result in losing large amounts of data.
Check for file systems on all the partitions of the disk before using the -p option. If a file system exists whose partition may be
destroyed, copy it to a backup medium. After you have changed the partitions, restore the backed up file system.
Restrictions
You must have superuser privileges to use
You can not shrink or change the offset of a partition with a file system mounted on it or with an open file descriptor on the entire par-
tition.
You can not change the offset of the a partition.
See Also
ioctl(2), disktab(5), fsck(8), mkfs(8), newfs(8)
Guide to System Disk Maintenance
chpt(8)