SAN Fabric Calculator
Size a Fibre Channel SAN: host-to-storage fan-out, switch count and buffer credits for long distance.
Inputs
Current Fan-out Ratio
10:1
Fan-out as a Number
10.00
Target Ports for the Target Ratio
30ports
FC Switches Required
6switches
Buffer-to-Buffer Credits per ISL
160credits
Fabric Ports to Provide
264ports
Step by step
Values used
Host (initiator) ports = 240 ports; Storage (target) ports = 24 ports; Target fan-out ratio (x:1) = 8 x; Ports per FC switch = 48 ports; ISL speed = 32 Gbps; Longest ISL distance = 10 km
SAN Fabric
fan-out = host ports ÷ storage ports; buffer credits ≈ distance km × ISL speed Gbps ÷ 2 for 2 KB frames.
Current Fan-out Ratio
= 10:1
Fan-out as a Number
= 10.00
Target Ports for the Target Ratio
= 30 ports
FC Switches Required
= 6 switches
Buffer-to-Buffer Credits per ISL
= 160 credits
Fabric Ports to Provide
= 264 ports
How it works
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.
Formula
SAN Fabric
fan-out = host ports ÷ storage ports; buffer credits ≈ distance km × ISL speed Gbps ÷ 2 for 2 KB frames.
- fan-out ratio
- Initiators sharing one target port — 8:1 to 12:1 is typical
- buffer credits
- Frames a port may send before an acknowledgement, one per 2 KB in flight
Frequently Asked Questions
How is SAN Fabric calculated?
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.
Why does SAN Fabric matter?
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.
What values do I need to enter?
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.