Volumes and Virtual Objects

Volume Manager Disks

When you place a physical disk under Volume Manager control, a Volume Manager disk (or VM Disk) is assigned to the physical disk. A VM Disk is under Volume Manager control and is usually in a disk group. Each VM disk corresponds to at least one physical disk. Volume Manager allocates storage from a contiguous area of Volume Manager disk space.

A VM disk typically includes a public region (allocated storage) and a private region where Volume Manager internal configuration information is stored.

Each VM Disk has a unique disk media name (a virtual disk name). You can supply the disk name or allow Volume Manager to assign a default name that typically takes the form disk##. See Figure 1-1 “Example of a VM Disk” which shows a VM disk with a media name of disk01 that is assigned to the disk devname.

Figure 1-1 Example of a VM Disk

Disk Groups

A disk group is a collection of VM disks that share a common configuration. A disk group configuration is a set of records with detailed information about related Volume Manager objects, their attributes, and their connections. The default disk group is rootdg (the root disk group).

You can create additional disk groups as necessary. Disk groups allow the administrator to group disks into logical collections. A disk group and its components can be moved as a unit from one host machine to another.

Volumes are created within a disk group. A given volume must be configured from disks in the same disk group.

Subdisks

A subdisk is a set of contiguous disk blocks. A block is a unit of space on the disk. Volume Manager allocates disk space using subdisks. A VM disk can be divided into one or more subdisks. Each subdisk represents a specific portion of a VM disk, which is mapped to a specific region of a physical disk.

The default name for a VM disk is disk## (such as disk01) and the default name for a subdisk is disk##-##. As shown in Figure 1-2 “Example of a Subdisk”, disk01-01 is the name of the first subdisk on the VM disk named disk01.

Figure 1-2 Example of a Subdisk

A VM disk can contain multiple subdisks, but subdisks cannot overlap or share the same portions of a VM disk. See Figure 1-3 “Example of Three Subdisks Assigned to One VM Disk”

Figure 1-3 Example of Three Subdisks Assigned to One VM Disk

Any VM disk space that is not part of a subdisk is free space. You can use free space to create new subdisks.

Volume Manager release 3.0 or higher supports the concept of layered volumes in which subdisk objects can contain volumes. For more information, see “Layered Volumes”.

Plexes

The Volume Manager uses subdisks to build virtual objects called plexes. A plex consists of one or more subdisks located on one or more physical disks. See Figure 1-4 “Example Plex With Two Subdisks”.

Figure 1-4 Example Plex With Two Subdisks

You can organize data on the subdisks to form a plex by using these methods:

Concatenation, striping (RAID-0), RAID-5, and mirroring (RAID-1) are described in “Virtual Object Data Organization (Volume Layouts)”.

	NOTE: You may need an additional license to use this feature.

Volumes

A volume is a virtual disk device that appears to applications, databases, and file systems like a physical disk device, but does not have the physical limitations of a physical disk device. A volume consists of one or more plexes, each holding a copy of the selected data in the volume. Due to its virtual nature, a volume is not restricted to a particular disk or a specific area of a disk. The configuration of a volume can be changed by using the Volume Manager user interfaces. Configuration changes can be done without causing disruption to applications or file systems that are using the volume. For example, a volume can be mirrored on separate disks or moved to use different disk storage.

The Volume Manager uses the default naming conventions of vol## for volumes and vol##-## for plexes in a volume. Administrators must select meaningful names for their volumes.

A volume can consist of up to 32 plexes, each of which contains one or more subdisks. A volume must have at least one associated plex that has a complete set of the data in the volume with at least one associated subdisk. Note that all subdisks within a volume must belong to the same disk group. See Figure 1-5 “Example of a Volume with One Plex”.

Figure 1-5 Example of a Volume with One Plex

Volume vol01 in Figure 1-5 “Example of a Volume with One Plex” has the following characteristics:

A volume with two or more data plexes is "mirrored" and contains mirror images of the data. See Figure 1-6 “Example of a Volume with Two Plexes”

Figure 1-6 Example of a Volume with Two Plexes

Each plex contains an identical copy of the volume data. For more information, see “Mirroring (RAID-1)”.

Volume vol06 in Figure 1-6 “Example of a Volume with Two Plexes” has the following characteristics:

Connection Between Volume Manager Virtual Objects

Volume Manager virtual objects are combined to build volumes. The virtual objects contained in volumes are: VM disks, disk groups, subdisks, and plexes. Volume Manager objects have the following connections:

The example in Figure 1-7 “Connection Between Volume Manager Objects” shows the connections between Volume Manager virtual objects and how they relate to physical disks. Figure 1-7 “Connection Between Volume Manager Objects” shows a disk group with two VM disks (disk01 and disk02). In disk01, there is a volume with one plex and two subdisks. In disk02, there is a volume with one plex and a single subdisk.

Figure 1-7 Connection Between Volume Manager Objects