Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Chapter 1, Understanding VERITAS Volume Manager
How VxVM Handles Storage Management
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The various types of virtual objects (disk groups, VM disks, subdisks, plexes and
volumes) are described in the following sections. Other types of objects exist in VERITAS
Volume Manager, such as data change objects (DCOs), and cache objects, to provide
extended functionality. These objects are discussed later in this chapter.
Disk Groups
A disk group is a collection of disks that share a common configuration, and which are
managed by VxVM (see “VM Disks” on page 11). A disk group configuration is a set of
records with detailed information about related VxVM objects, their attributes, and their
connections. A disk group name can be up to 31 characters long.
In releases prior to VxVM 4.0, the default disk group was rootdg (the root disk group). For
VxVM to function, the rootdg disk group had to exist and it had to contain at least one
disk. This requirement no longer exists, and VxVM can work without any disk groups
configured (although you must set up at least one disk group before you can create any
volumes of otherVxVM objects). For more information about changes to disk group
configuration, see “Creating and Administering Disk Groups” on page 131.
You can create additional disk groups when you need them. Disk groups allow you to
group disks into logical collections. A disk group and its components can be moved as a
unit from one host machine to another. The ability to move whole volumes and disks
between disk groups, to split whole volumes and disks between disk groups, and to join
disk groups is described in “Reorganizing the Contents of Disk Groups” on page 155.
Volumes are created within a disk group. A given volume and its plexes and subdisks
must be configured from disks in the same disk group.
VM Disks
When you place a physical disk under VxVM control, a VM disk is assigned to the
physical disk. A VM disk is under VxVM control and is usually in a disk group. Each VM
disk corresponds to one physical disk. VxVM allocates storage from a contiguous area of
VxVM disk space.
A VM disk typically includes a public region (allocated storage) and a private region where
VxVM internal configuration information is stored.
Each VM disk has a unique disk media name (a virtual disk name). You can either define a
disk name of up to 31 characters, or allow VxVM to assign a default name that takes the
form diskgroup##, where diskgroup is the name of the disk group to which the disk
belongs (see “Disk Groups” on page 11).