Veritas™ Volume Manager Administrator's Guide

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.

You must place disks in one or more disk groups before VxVM can use the disks for volumes. It is important that you locate data related to particular applications or users on an identifiable set of disks. When you need to move these disks, this lets you move only the application or user data that should be moved. The disk group also provides a single object to move, rather than specifying all objects within the disk group individually.

As system administrator, you can create additional disk groups to arrange your system's disks for different purposes. Many systems only use one disk group, unless they have a large number of disks. You can initialize, reserve, and add disks to disk groups at any time. You do not have to add disks to disk groups until the disks are needed to create VxVM objects.

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

For more information about CDS, see the Veritas Storage Foundation Advanced Features Administrator's Guide.

When you add a disk to a disk group, you name that disk (for example, mydg02). This name identifies a disk for operations such as creating or mirroring a volume. 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. You can replace disks 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. If you have large disk groups that are expected to contain more than several hundred disks and VxVM objects, you should set up disks 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 is approximately 256 bytes, you can use the configuration database copy size to estimate the number of records that you can create in a disk group. You can obtain the copy size in blocks from the output of the vxdg list diskgroup command. It is 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 value of the free parameter indicates the amount of remaining free space in the configuration database.

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

Before Veritas Volume Manager (VxVM) 4.0, a system installed with VxVM was configured with a default disk group, rootdg. This group 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. Now, you do not have to name any disk group rootdg. If you name a disk group rootdg, it has no special properties because of this name.

Additionally, before 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.