In order to calculate the maximum possible throughput, we combine the two phases 2 and 3 (propagation delay server-storage and storage-server) to form what is known as the round trip time (RTT). The time duration (Completion Time CT) for a single complete I/O is then:
CT = TD + RTT + additional delays
(Under “additional delays” all delays are summarized, which we have assumed to be non-existent for the sake of simplicity.)
The throughput for a single I/O results from the I/O size (IOsize) divided by the time required for the I/O (CT):
Throughput = IOsize / CT = IOsize / ( TD + RTT + additional delays ) <= IOsize / ( TD + RTT )
The actual throughput can therefore at best be IOsize / (TD + RTT), if there is no delay in processing or forwarding at any point. With a given I/O size, the maximum achievable throughput depends on the transmission delay (TD) and the round-trip time (RTT).
This is shown graphically below for round-trip times up to 2 ms and various FC technologies.
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