Size a Fibre Channel SAN: host-to-storage fan-out, switch count and buffer credits for long distance.
Fan-out compares how many host ports share each storage port, which is what determines queue depth contention on the array. Over distance, a Fibre Channel port must hold enough buffer credits to keep the light path full — roughly half a credit per kilometre per gigabit. Too high a fan-out starves hosts during backup windows, and too few buffer credits on a metro ISL cuts throughput to a fraction of line rate with no error counters to show why.
SAN Fabric
fan-out = host ports ÷ storage ports; buffer credits ≈ distance km × ISL speed Gbps ÷ 2 for 2 KB frames.
fan-out = host ports ÷ storage ports; buffer credits ≈ distance km × ISL speed Gbps ÷ 2 for 2 KB frames. Fan-out compares how many host ports share each storage port, which is what determines queue depth contention on the array. Over distance, a Fibre Channel port must hold enough buffer credits to keep the light path full — roughly half a credit per kilometre per gigabit.
Too high a fan-out starves hosts during backup windows, and too few buffer credits on a metro ISL cuts throughput to a fraction of line rate with no error counters to show why.
This calculator takes 6 inputs: Host (initiator) ports, Storage (target) ports, Target fan-out ratio (x:1), Ports per FC switch, ISL speed, Longest ISL distance. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.