• Without dynamic load-balancing, traffic can be distributed unevenly across:
– HBA ports within a given host – ISLs and redundant fabrics – Storage ports
Fabric A
120MB/s
Fabric B
200MB/s congested
This example illustrates how uneven distribution of traffic on the host ports can ultimately cause congestion in the fabric and on storage ports. Two hosts with two ports each are connected to a dual-ported storage subsystem through redundant fabrics. All LUNs are accessible via both storage ports. However, each LUN can only be accessed via one HBA port— the host can’t load-balance across both HBA ports. In this case the administrator has manually assigned paths on each host. On the upper host, one path is assigned to the green LUN (60MB/s) and the other path is assigned to the orange LUN (120MB/s). On the lower host, one path is assigned to the blue LUN (60MB/s) and the other path is assigned to the purple LUN (120MB/s).
The unequal traffic loads on each path result in congestion on one storage port.
This situation could easily be corrected by simply swapping the data paths on one of the hosts. In a large SAN, however, manual load-levelling would be a very complex task. It would also have to be adjusted constantly as traffic loads shifted over time.