Host groups are useful if you have host clusters, each of which contains many hosts and each host contains a few initiators. If a host cluster is not defined in the Oracle FS System GUI as a host group, when you want to map a LUN to the cluster, you need to map each SAN host to the LUN one at a time. Furthermore if you need to move a host to a different cluster, you must manually update each LUN mapping, also one at a time.
A more efficient method is to define the cluster as a host group, and then assign the SAN hosts to the host group. When you subsequently move a host from one host group to another, all of the initiators associated with that host inherit the LUN mapping associated with that host group.
A host can belong to only one host group.
You can map an unlimited number of hosts to a host group.
A host group can have zero or more mappings.
You can map a LUN to either a host or a host group.
If a host group has mappings, then all hosts in the host group will have all of the mappings of the host group, but any given host can also have other mappings. No mappings can conflict.
When assigning a host with mappings to a host group without mappings, you have the option to migrate mappings on the host to the host group, making those mappings available to all hosts in the group, not just the single host.