Why go to 4 Gbit Fibre Channel if I'm not already using all of my 1 Gbit or 2 Gbit bandwidth?

Depending on your needs, there may not be a compelling argument to switch to 4 Gbit...

Depending on your needs, there may not be a compelling argument to switch to 4 Gbit. People, even the vendors, get too focused on the bandwidth characteristics of Fibre Channel (FC). There's an obsession with bandwidth in general.

SAN information
Fibre Channel Arbitrated Loop or switched disk?

ISCSI vs. FC performance
In reality, many common business applications do require some level of bandwidth but they're also very time sensitive, demanding low response time (low latency) and those applications tend to handle a larger number of smaller I/O operations. In that case, you don't see large bandwidth, but there is a lot of activity. Tasks like video streaming or seismic telemetry -- large sequential data transfers, are not so time sensitive, but do emphasize streaming across high bandwidth. Generally speaking, the mainstream commercial application may not use the entire 1 Gbit or 2 Gbit bandwidth but the IOPS and response time on the network can really make or break the application -- it's not all about the bandwidth.

Going to a faster bandwidth may yield benefits in reduced latency, allowing time-sensitive applications to run faster. Similarly, the need to aggregate might allow a switch to fan in several slower servers to a faster 4 Gbit pipe to a 4 Gbit storage device.

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