28Understanding Veritas Volume Manager

How VxVM handles storage management

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 28). 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 165.

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 195.

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 small 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 28).

Figure 1-6shows a VM disk with a media name of disk01 that is assigned to the physical disk devname.