Typically directors are used in high-availability mission-critical SANs but also as core switches where the downtime of a switch has a high impact. In a core-edge SAN, downtime of the core switch will impact all hosts as the switch connects to all the storage ports. Some SAN users choose to implement directors across their entire environment to guarantee a high level of availability however this is a tradeoff against the increased cost of director ports. You should recommend directors for core switches and for any applications which need all day, every day availability.
This was first published in January 2007