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SCSI is a multi-threaded I/O interface: it can process multiple I/O requests simultaneously. The SCSI bus remains free until a device has a command, data to pass, or status to send via the bus. When this occurs, the transaction is orchestrated by the SCSI host adapter. A requesting device can disconnect from the SCSI bus until it has information to send. This allows multiple devices to process requests simultaneously, providing an effective way to overlap the mechanical drive delays required in locating data. It is also an effective way to share the bus bandwidth among all connected devices. With SCSI, the throughput of all drives can be combined for up to 80MB per second.

SCSI's multi-threading doesn't just provide benefits in multiple peripheral environments, it also allows for multiple requests to be processed simultaneously by one device in a single application. SCSI can prioritize commands and allows the drive to optimize requests. Command queuing can be especially valuable in workstations with a single hard disk drive.

Not only is SCSI an intelligent interface, SCSI drives have the best mechanical performance ratings. SCSI drives on the market today have spindle speeds of up to 10,000 RPM.

SCSI vs. Ultra DMA
SCSI, the traditional PC I/O powerhouse, is facing challenges from the Ultra DMA interface. Ultra DMA has established itself as the standard desktop drive interface. However, benchmark tests on Ultra2 SCSI drives reveal significant performance improvements over Ultra DMA.

Ultra DMA remains a single-threaded I/O interface with the ability to execute only one I/O request at a time, which restricts total data throughput speeds. The drives themselves become the bottleneck, prohibiting UDMA from performing transfer rates at its theoretical potential. As a result, even doubling the maximum transfer rate to 33 MB per second does little to increase overall performance. The limitations of the drive-based bottleneck still remain.

Ultra DMA performance does not benefit from improvements in the interface. Instead, performance improvements come from an increase in the data rate of the drives themselves.

Ultra DMA does not support overlapped seeks, bus bandwidth sharing or command queuing because of its single-threaded nature.

In comparison, the best 10,000-RPM SCSI drive is significantly faster than the best Ultra DMA drive. In absolute terms, a 10,000-RPM Ultra2 SCSI drive can sustain transfers at 12.6 MB per second. SCSI drives spinning at up to 10,000 RPM have the most advanced mechanics, providing for the fastest data exchange with the drive and across the bus.

While Ultra DMA provides the minimum functionality and performance at the lowest cost to meet the I/O requirements of the non-demanding desktop PC user, SCSI is built for the performance I/O requirements of workstations, file servers, and sophisticated applications.

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