Host groups and LU path configuration (Fibre Channel)

After open-system hosts and the storage system are physically connected by cables, hubs, and so on, you can establish I/O paths between the hosts and the logical volumes. This defines which host can access which logical volume. Logical volumes that can be accessed by open-system hosts are referred to as logical units (LUs). The paths between the open-system hosts and the LUs are referred to as LU paths.

Before defining LU paths, you must classify server hosts by host groups. For example, if Linux hosts and Windows hosts are connected to the storage system, you must create one host group for the Linux hosts and another host group for the Windows hosts. Then, you must register the host bus adapters of the Linux hosts in the Linux host group, and you must also register the host bus adapters of the Windows hosts in the Windows host group.

A host group can contain only hosts that are connected to the same port. For example, if two Windows hosts are connected to port 1A and three Windows hosts are connected to port 1B, you cannot register all five Windows hosts in one host group. You must register the two Windows hosts on port 1A in one host group, and then register the three Windows hosts on port 1B in another host group.

After server hosts are classified into host groups, you associate the host groups with logical volumes. The following figure illustrates LU path configuration in a Fibre Channel environment. The figure shows host group hg-lnx associated with three logical volumes (00:00:00, 00:00:01, and 00:00:02). LU paths are defined between the two hosts in the hg-lnx group and the three logical volumes.

You can define paths between a single server host and multiple LUs. The figure shows that each of the two hosts in the host group hg-lnx can access the three LUs.

You can also define paths between multiple server hosts and a single LU. The figure shows that the LU identified by the LDKC:CU:LDEV number 00:00:00 is accessible from the two hosts that belong to the hg-lnx host group.

The figure also shows that the LUs associated with the hg-lnx host group are addressed by numbers 0000 to 0002. The address number of an LU is referred to as a LUN (logical unit number). When software manipulates LUs, the software use LUNs to specify the LUs to be manipulated.

You can add, change, and delete LU paths when the system is in operation. For example, if new disks or server hosts are added to your storage system, you can add new LU paths. If an existing server host is to be replaced, you can delete the LU paths that correspond to the host before replacing the host. You do not need to restart the system when you add, change, or delete LU paths.

If a hardware failure (for example, CHA failure) occurs, there is a chance that some LU paths are disabled and some I/O operations are stopped. To avoid such a situation, you can define alternate LU paths; if one LU path fails, the alternate path takes over the host I/O.