Fabric versus Loop Fibre Channel

By Rick Cook

In the last year a number of manufacturers have begun offering Fibre Channel fabric as well as Fibre Channel arbitrated loop (FC-AL) for SAN applications. Choosing the right one can make a big difference in the price/performance of your SAN.

"Fabric" is the fibre channel equivalent of a non-blocking crossbar switch. It provides high-speed any-to-any connectivity at a price per port which is currently about three times that of FC-AL. (FC-AL offers point-to-point connectivity with some switching and partitioning capability through FC-AL hubs.) As far as current prices are concerned, you can expect to pay about $1,000 per port for Fibre Channel fabric versus about $300 per port for FC-AL, according to the industry conventional wisdom.

According to Skip Jones, president of the Fibre Channel Industry Association (FCIA), distance and complexity are the keys to choosing between FC-AL and fabric. "Complex and spread-out topologies make a good case for switching," Jones says.

If the SAN includes more than a couple of servers and/or more than a couple of storage units and if expandability is important, fabric is attractive in spite of the higher cost, Jones says. Other factors favoring fabric include large distances between parts of the SAN, as in a remote mirroring application, and complex subdivision of the network.

FC-AL works best with simpler layouts, such as one or two servers and storage units connected directly

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and only a limited amount of partitioning, Jones says.

For more information on Fibre Channel, go to the FCIA's Web site at: http://www.fibrechannel.org .

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.

This was first published in July 2000

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