Directors are SAN switches on steroids. They offer more ports, high reliability and usually a good deal more intelligence than regular switches. They also typically have special features that make managing large, complex data flows easier.
Ports and speeds are both moving targets when it comes to directors. Currently, directors offer a minimum of 64 ports on an individual switch and scale to as high as 256 ports (IBM Corp.'s TotalStorage SAN256M) in a single switch. Modern director switches support at least 2 Gbps connections and can be as fast as 10 Gbps on their internal backbone. Not surprisingly, many manufacturers first introduce 4 Gbps connectivity in their directors and related equipment. Both port numbers and speeds of directors are increasing as manufacturers push the limits of the technology to meet the needs of ever larger, ever-more-complex SANs. Most director-class switches are also non-blocking so all their ports can operate at full speed.
Since directors sit at the center of SANs, they are designed for extremely high reliability. Everything from the power supplies to control processors is typically hot-swappable.
Directors are significantly more expensive than conventional SAN switches (although on a per-port basis the differential is much less), and are best employed in the most demanding applications. They are designed as cores in core/edge SANs. As a result, they seldom connect directly to servers or storage devices. (In fact, one of the things you usually can't do with a director is use it to support FC-AL arbitrated loop connections.) Directors are also well-suited for tying together SAN islands -- independent SANs that need to be integrated with very high bandwidth and throughput.
Directors are also well-suited for connecting a backup network with multiple tape libraries or backup disk arrays with the main SAN. The combination of high throughput and port count with high reliability allows backing up the supported SAN through the director in the minimum time and with the minimum performance hit on the SAN.
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About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years, he has been a freelance writer specializing in storage and other computer issues.