Zoning is a powerful, flexible method for dividing up a SAN and assigning its devices. However, zoning is only part of the overall schema for structuring a SAN. ANSI standards actually define a hierarchy of organization within a zoned SAN, starting with the member devices and leading up to the zone set.
Logically, zone sets are a step above zones. That is, a zone set is composed of zones, which are composed of members -- devices such as switches and disk arrays. While a SAN can have many zone sets, only one zone set -- often consisting of multiple zones -- can be active at a time. In most installations, however, zone sets can be changed without bringing down the SAN. Although the SAN doesn't have to be shut down to change zone sets, the SAN should be idle when the zone set is actually changed.
For SAN managers, the major advantage of using zone sets is flexibility. Zones can be members of different zone sets for different purposes, such as backup and dynamic reconfiguration. They can also be used for testing and maintenance of some of the devices in the SAN without affecting the other devices.
The Fibre Channel Industry Association (FCIA) Web site has a number of white papers discussing zone sets and their uses; among them are "An Introduction to Zoning" by McData and "Zoning: The Key To Flexible SAN Management."
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.