Storage speed is the most important performance factor in servers. After a period of relative stasis in storage interfaces, three technologies are emerging:
Ultra SCSI
Considered by some an interim step toward serial interfaces, Ultra SCSI represents the latest improvement to the decade-old technology. Ultra SCSI uses the same command set as its predecessors (i.e., SCSI-2 in various configurations) and the same parallel ribbon cable. But by doubling the clock rate of the bus, Ultra SCSI offers burst throughput of up to 20 MBps for 8-bit connections and 40 MBps for 16-bit connections. SCSI champions suggest that one final ratcheting of parallel SCSI technology might someday yield an 80-MBps rating.
S
erial Storage Architecture
IBM's SSA is a serial implementation of the SCSI-2 command set, offering 20 MBps of bidirectional throughout, for a total capacity of 40 MBps. (Plans call for capacity to rise to 200 MBps in 1996.) SSA is implemented not as a bus but rather as a series of independent hops among up to 126 hot-pluggable, self-configuring devices. IBM intends it as a low-cost universal solution for local storage. Early supporters include Adaptec, Conner, Future Domain, Micropolis, NCR, and Western Digital.
Fibre Channel-Arbitrated Loop
Based on work done by the IEEE's Fibre Channel committee, FC-AL is another serial interface that does not, despite its name, require a fiber connection. Based on SCSI-3 commands, it offers data transfer rates of up to 200 MBps and supports 126 hot-pluggable, self-configuring devices on a loop. Promoters argue that FC-AL tolerates drive failures better and, because of its close ties to Fibre Channel, could be used as a unive
rsal interconnect for both systems and storage. Supporters include Adaptec, BusLogic, Hewlett-Packard, Q-Logic, Quantum, NCR, and Seagate. SSA partisans fault FC-AL for being more expensive and consuming more power. IBM supports both interfaces: SSA for storage and Fiber Channel for system interconnect.
Storage Interfaces
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Faster Storage Is Key
