Disk Groups

Disks are organized by the Volume Manager into disk groups. A disk group is a named collection 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 the Volume Manager installed has the default disk group, rootdg. By default, operations are directed to the rootdg disk group. The system administrator can create additional disk groups as necessary. 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 Volume Manager objects. Disks can be initialized, reserved, and added to disk groups later. However, at least one disk must be added to rootdg for you to do the Volume Manager installation procedures.

Even though the rootdg is the default disk group, it is not the root disk group. 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 to log in. A major portion of a private region is space for a disk group configuration database containing records for each Volume Manager object in that disk group. Because each configuration record takes up 256 bytes (or half a block), the number of records that can be created in a disk group is twice the configuration database copy size. The copy size can be obtained from the output of the command vxdg list diskgroupname.

You may wish to add a new disk to an already established disk group. For example, the current disks may have insufficient space for the project or work group requirements, especially if these requirements have changed. You can add a disk to a disk group by following the steps required to add a disk. See Chapter 4 “Disk Tasks”.