SAN Fabric Calculator
Size a Fibre Channel SAN: host-to-storage fan-out, switch count and buffer credits for long distance.
About SAN Fabric Calculator
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
Worked example
With the default values below, SAN Fabric Calculator returns Current Fan-out Ratio of 10:1. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Host (initiator) portsaccepts 1–20,000 ports
- 240 ports
- Storage (target) portsaccepts 1–5,000 ports
- 24 ports
- Target fan-out ratio (x:1)accepts 1–40 x
- 8 x
- Ports per FC switchaccepts 8–512 ports
- 48 ports
- ISL speedaccepts 2–128 Gbps
- 32 Gbps
- Longest ISL distanceaccepts 0–500 km
- 10 km
Results
- 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
Step by step
- 1Values used
- 2SAN Fabric
- 3Current Fan-out Ratio= 10:1
- 4Fan-out as a Number= 10.00
- 5Target Ports for the Target Ratio= 30 ports
- 6FC Switches Required= 6 switches
- 7Buffer-to-Buffer Credits per ISL= 160 credits
- 8Fabric Ports to Provide= 264 ports
Assumptions and limits
- The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.