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About 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.

In releases of Veritas Volume Manager (VxVM) prior to 4.0, a system installed with VxVM was configured with a default disk group, rootdg, that had to contain at least one disk. By default, operations were directed to the rootdg disk group. From release 4.0 onward, VxVM can function without any disk group having been configured. Only when the first disk is placed under VxVM control must a disk group be configured. There is no longer a requirement that you name any disk group rootdg, and any disk group that is named rootdg has no special properties because of this name.

See "Specification of disk groups to commands" on page 176.

  Note   Most VxVM commands require superuser or equivalent privileges.

Additionally, prior to VxVM 4.0, some commands such as vxdisk were able to deduce the disk group if the name of an object was uniquely defined in one disk group among all the imported disk groups. Resolution of a disk group in this way is no longer supported for any command.

The Cross-platform Data Sharing (CDS) feature of Veritas Volume Manager allows you to move VxVM disks and objects between machines that are running under different operating systems. Disk groups may be made compatible with CDS.

See the Veritas Storage Foundation Cross-Platform Data Sharing Administrator's Guide.

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 can be initialized, reserved, and added to disk groups at any time. Disks need not be added to disk groups until the disks are needed to create VxVM objects.

When a disk is added to a disk group, it is given a name (for example, mydg02). This name identifies a disk for operations such as volume creation or mirroring. The name also relates directly to the underlying physical disk. If a physical disk is moved to a different target address or to a different controller, the name mydg02 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 disk groups that contain many disks and VxVM objects causes the private region to fill. In the case of large disk groups that are expected to contain more than several hundred disks and VxVM objects, 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 approximately 256 bytes, the number of records that can be created in a disk group can be estimated from the configuration database copy size. The copy size in blocks can be obtained from the output of the command vxdg list diskgroup as the value of the permlen parameter on the line starting with the string "config:". This value is the smallest of the len values for all copies of the configuration database in the disk group. The amount of remaining free space in the configuration database is shown as the value of the free parameter.

See "Displaying disk group information" on page 179.

One way to overcome the problem of running out of free space is to split the affected disk group into two separate disk groups.

See "Reorganizing the contents of disk groups" on page 205.

See "Backing up and restoring disk group configuration data" on page 223.