To support multipathing, your host typically has two or more HBAs available. This configuration supplements the SAN multipathing configuration. Generally, the SAN multipathing provides one or more switches in the SAN fabric and one or more storage processors on the storage array device itself.
In the following illustration, multiple physical paths connect each server with the storage device. For example, if HBA1 or the link between HBA1 and the FC switch fails, HBA2 takes over and provides the connection. The process of one HBA taking over for another is called HBA failover.
Similarly, if SP1 fails or the links between SP1 and the switches breaks, SP2 takes over. SP2 provides the connection between the switch and the storage device. This process is called SP failover. VMware ESXi supports both HBA and SP failovers.