The first part of our process is to decide whether or not we actually need a SAN at all. If the database will remain relatively small, capable of residing easily on local hard drives for several years, we don't need a SAN just yet.
While it might seem arbitrary, setting 3 TB as a cutoff for local storage comes with a few justifications. First, consider the local drives. Even if they were capable of saturating a 6 Gbps disk controller, 3 TB would require over an hour to transfer to another local storage device. If that wasn't a bottleneck, there is still the network. With a 10 Gbps NIC and assuming no overhead, that's 40 minutes of transfer at full speed.
That directly affects speed of backups, synchronization, emergency data ...
