HP Veritas Volume Manager 5.0 -UX 11i v3 manual Private and shared disk groups

Administering cluster functionality

Overview of cluster volume management

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Private and shared disk groups

Two types of disk groups are defined:

Private disk group	Belongs to only one node. A private disk group can only be
	imported by one system at a time. Disks in a private disk group
	may be physically accessible from one or more systems, but
	access is restricted to one system only. The boot disk group
	(usually aliased by the reserved disk group name bootdg) is
	always a private disk group.
Shared disk group	Can be shared by all nodes. A shared (or cluster-shareable) disk
	group is imported by all cluster nodes. Disks in a shared disk
	group must be physically accessible from all systems that may
	join the cluster.

In a cluster, most disk groups are shared. Disks in a shared disk group are accessible from all nodes in a cluster, allowing applications on multiple cluster nodes to simultaneously access the same disk. A volume in a shared disk group can be simultaneously accessed by more than one node in the cluster, subject to licensing and disk group activation mode restrictions.

You can use the vxdg command to designate a disk group as cluster-shareable as described in “Importing disk groups as shared” on page 423. When a disk group is imported as cluster-shareable for one node, each disk header is marked with the cluster ID. As each node subsequently joins the cluster, it recognizes the disk group as being cluster-shareable and imports it. As system administrator, you can also import or deport a shared disk group at any time; the operation takes place in a distributed fashion on all nodes.

Each physical disk is marked with a unique disk ID. When cluster functionality for VxVM starts on the master, it imports all shared disk groups (except for any that have the noautoimport attribute set). When a slave tries to join a cluster, the master sends it a list of the disk IDs that it has imported, and the slave checks to see if it can access them all. If the slave cannot access one of the listed disks, it abandons its attempt to join the cluster. If it can access all of the listed disks, it joins the cluster and imports the same shared disk groups as the master. When a node leaves the cluster gracefully, it deports all its imported shared disk groups, but they remain imported on the surviving nodes.

Reconfiguring a shared disk group is performed with the cooperation of all nodes. Configuration changes to the disk group happen simultaneously on all nodes and the changes are identical. Such changes are atomic in nature, which means that they either occur simultaneously on all nodes or not at all.

Whether all members of the cluster have simultaneous read and write access to a cluster-shareable disk group depends on its activation mode setting as discussed in “Activation modes of shared disk groups.” The data contained in a