Managing host clusters

Edit online

A host cluster is a group of logical host objects that can be managed together. For example, you can create a volume mapping that is shared by every host in the host cluster.

The system uses internal protocols to manage access to the volumes and ensure consistency of the data. Host objects that represent hosts can be grouped in a host cluster and share access to volumes. A host can only be in one host cluster at a time. New volumes can also be mapped to a host cluster, which simultaneously maps that volume to all hosts that are defined in the host cluster.

When you create a host cluster, you can specify the hosts to include in the cluster. You can also create an empty host cluster and add hosts later. If you select hosts to add to the host cluster, all of the common private volume mappings for the hosts become shared mappings for the host cluster. However, you can also exclude specific volumes from being shared. The private volume mappings are not shared with any other hosts in the host cluster. A host can be a member of only one host cluster at a time.

With shared mapping, volumes are mapped on a host cluster basis. The volumes are shared by all of the hosts in the host cluster, if there are no Small Computer System Interface (SCSI) LUN conflicts among the hosts. Volumes that contain data that is needed by other hosts are examples of a shared mapping.

If any mappings contain SCSI LUN conflicts, the operation fails. The management GUI displays all mappings with SCSI LUN ID conflicts. SCSI LUN conflicts can occur if multiple volumes are mapped with the same SCSI LUN ID or if same volume is mapped to multiple SCSI LUN IDs.

Note: When adding hosts to a host cluster, if you selected hosts with volumes that have SCSI ID conflicts, the system does not add these mappings to the host cluster. A SCSI LUN ID conflict occurs when multiple hosts are mapped to the same volume but with different SCSI IDs. In this case, a shared mapping is not created because the system does not allow a volume to be mapped more than once to the same host. The Summary page lists all volumes that contain conflicts and the system retains these mappings as private mappings to the original hosts.

If throttles are configured on any of the selected hosts, the throttles must be removed to be included in the host cluster. Throttles can be applied only to the host cluster and not individual hosts within the cluster. If you choose not to remove the throttles from the host, the host is excluded from the host cluster.

When a host is removed from a host cluster, you can convert the shared mappings of the host cluster to private mappings or remove all shared mappings from the host. If shared mappings are converted to private mappings, volume access is not removed since the volume remains mapped to the host and its SCSI ID remains unchanged. If you choose to remove the shared mappings when you remove a host, only the private mappings for that host are retained when it is removed from the host cluster. When you remove the shared mapping, hosts cannot access those volumes that were mapped as shared mappings to the host cluster. If the host that is being removed is the last host in the host cluster, then the -force parameter must be used because removing the last host also removes all shared mappings from the host cluster.

You can use several tools to manage host clusters, including the management GUI, and the command-line interface (CLI). To work with host clusters in the management GUI, select Hosts > Host Clusters.