Each SCSI device willappear only on one SCSI bus. IN FC, it is not true and because of multiple HBA in the same host, it could have more than 1 path to the same device. SAN switch is closer to Ethernet router than switches. Ethernet switch uses spanning tree to detect loop and disable redundant path. Ethernet router uses a routing protocol such as OSPF to ahndle multiple path. In FC, FSPS (FC Shortest Path First) to load balanceSAN traffic and failover.
Because each device on the SAN will appear as a SCSI ID on each HBA connected tot he same, a system with multiple HBA connected to the SAN will actually think that each device on each path is a separte device. A logical layer needs to be inserted to mask these paths and present the OS with the appearance of a single SCSI connection. This is achieved by multipathing software runs on each server.
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