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New FC-AL technology ushers in "SBOD"

Many SANs use FC-AL (Fibre Channel Arbitrated Loop) within the storage arrays to connect the actual storage devices (disks or tapes) to the controllers. FC-AL is a robust, relatively inexpensive technology, but essentially daisy-chains up to 126 drives on one controller, which in turn limits performance and complicates diagnosing problems.

To get better performance, a number of array manufacturers are adopting a technology called Inspeed from Vixel (recently purchased by

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Emulex) which puts in what amounts to an FC-AL switch and allows the controller to establish point-to-point connections between a storage device and the controller. This provides higher bandwidth and higher overall performance. Vixel calls the resulting architecture a Switched Bunch of Disks (SBOD), playing off the name of JBOD (Just a Bunch of Disks) storage architecture.

While Inspeed doesn't offer the highest possible performance, it does offer higher performance than regular FC-AL arrays at a price that is only somewhat higher than FC-AL. This makes it attractive to for middle-range storage solutions that need more bandwidth. A number of companies, including Fujitsu and Sun have adopted the Vixel technology for mid-range storage arrays.

The technique is described in a white paper at Vixel.


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 December 2003

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