VERITAS Volume Manager 4.1 Administrator's Guide

Chapter 4 151
4 Creating and Administering
Disk Groups
Introduction
This chapter describes how to create and manage disk groups. Disk
groups are named collections of disks that share a common
configuration. Volumes are created within a disk group and are
restricted to using disks within that disk group.
A system with Volume Manager (VxVM) installed has a default disk
group configured, rootdg. By default, operations are directed to the
rootdg disk group. As system administrator, you can create additional
disk groups to arrange your system’s disks for different purposes. Many
systems do not use more than one disk group, unless they have a large
number of disks. Disks are not added to disk groups until the disks are
needed to create VxVM objects. Disks can be initialized, reserved, and
added to disk groups later. However, at least one disk must be added to
rootdg when you initially configure VxVM after installation.
NOTE Even though rootdg is the default disk group, it does not contain the root
disk. In the current release the root volume group is always under LVM
control.
When a disk is added to a disk group, it is given a name (for example,
disk02). This name identifies a disk for volume operations: volume
creation or mirroring. This name relates directly to the physical disk. If a
physical disk is moved to a different target address or to a different
controller, the name disk02 continues to refer to it. Disks can be replaced
by first associating a different physical disk with the name of the disk to
be replaced and then recovering any volume data that was stored on the
original disk (from mirrors or backup copies).
Having large disk groups can cause the private region to fill. In the case
of larger disk groups, disks should be set up with larger private areas. A
major portion of a private region provides space for a disk group
configuration database that contains records for each VxVM object in
that disk group. Because each configuration record takes up 256 bytes (or
one quarter of a block), the number of records that can be created in a
disk group can be estimated from the configuration database copy size.