The Belated Blessings of Fibre

When fibre channel technology was introduced a couple years ago, I thought that fibre channel would be much more dominant than it is. Although it's important as a developing trend, fibre channel is still in its infancy as a market force. SCSI has kept improving so that it now offers competitive throughput, and people are still buying it. More than 90 percent of hard drives shipped are SCSI drives, and only two major manufacturers—Seagate and IBM—are shipping fibre channel drives.

Why is fibre channel a better technology? For one, it handles far longer cables, up to 10 kilometers at speeds up to 100MBps—200MBps in full-duplex mode. Fibre channel can make an ordinary disk on a server 6 miles away appear as if it's local storage. SCSI cables can be no longer than 12 meters in their most recent implementation. Fibre cables are thin; SCSI cables are bulky. With fibre, you can put 126 devices on a bus; with SCSI, you can put only 15. You can hot-plug devices with fibre because it's serial technology. You'd never try to hot-plug devices with SCSI because it's a bus. In addition, dual-loop fibre offers redundancy, which eliminates the single point of failure, a major differentiator between the two.

Nonetheless, if you take a look inside high-end storage devices, such as EMC's Symmetrix system, you'll find that they're loaded with SCSI drives. EMC does offer a fibre channel interface, and 60 percent of Symmetrix’s systems ship with one. Within a year, EMC expects that number to approach 100 percent. If you buy an expensive Storage Area Network (SAN) device, you might assume that you'll get a full fibre channel implementation. Don't count on it. The large storage system vendors plan to go to fibre as soon as the market is ready for it.

In the SCSI community, congratulations are in order for the recent ratification of the Ultra3 SCSI specification. When you add double-transition clocking to Ultra3, you get 160MBps SCSI. The specification adds five features: cyclical redundancy check (CRC), domain validation, packetization, quick arbitration and selection (QAS), and better connection-speed negotiation with fewer transmission errors. Insiders expect packetization to lower overhead traffic on a busy network up to 50 percent. When you add one of these five features to a disk drive, you can use the Ultra160 qualification—the name of the standard—and charge more.

SCSI continues to sell well because people are comfortable with the technology, and connectivity and interoperability issues still plague fibre channel. You can expect SCSI to continue to be strong, with fibre channel making its way into the market from the top down.

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