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SATA Takes on SCSI

SCSI: The Grizzled Veteran

Since the late 1980s, SCSI has dominated the high-performance storage market for workstations and servers. At the heart of the SCSI controller is a dedicated ASIC that manages all communications with its attached devices. This means a SCSI controller and the devices connected to its bus can function almost independently from the host system. As a result, disk-intensive applications using IDE/ATA in the early 1990s could have required 10 to 15 times the CPU resources used by SCSI under the same circumstances.

Although SCSI's bandwidth and flexibility have improved over the years, the modern Ultra320 SCSI interface is still based on the original parallel-bus architecture, where all the devices on a channel share a common ribbon cable that is terminated after the last device. The Ultra320 SCSI interface supports a transfer rate of up to 320 MB per second and offers a full range of server-class controller cards and disk drives. SCSI hard drives are available in capacities of up to 300 GB, and the smaller, highest-performance SCSI drives operate at a mind-boggling 15,000 RPM. For the small RAID arrays needed for economical servers and workstations, SCSI had no serious competition until the appearance of quality SATA drives and controllers over the past few years.

SATA: The Contender

Serial ATA was developed as the logical successor to IDE/ATA, which arose in the mid-1980s as an alternative to controller-dependent MFM (Modified Frequency Modulation) and RLL (Run-Length Limited) drive systems. Like SCSI, the IDE/ATA interface was based on a parallel connection, but that's where the similarities ended. IDE/ATA was limited to two devices per channel, configured in a master-slave relationship. Rather than being an independent bus controlled by a dedicated ASIC, IDE/ATA depended solely on the system's CPU to handle storage requests. As a result, disk-intensive applications demanded substantial system resources--for some tasks, as much as 90 percent CPU utilization compared with SCSI's 5 percent.

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