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Command Overhead and Multiple Device Considerations

When looking at interface specifications or raw transfer rates, don't forget that each transfer from the hard disk requires that a set of commands be sent to the hard disk to select what will be sent and to control the transfer. These take a certain amount of time, and this command overhead can and will reduce performance.

In comparing the SCSI and IDE/ATA interfaces, command overhead is an important consideration. SCSI is a much more intelligent and capable bus, but it is also more complex, which means more work must be done to set up a transfer. This means that SCSI can be slower than IDE/ATA in a single-user, single-tasking environment, even though it can be much faster and more capable in a machine that is supporting multiple users or multiple devices on the same bus. SCSI shines when you need to use multiple devices on a single bus, where IDE/ATA starts to become cumbersome. See here for more on the eternal IDE vs. SCSI question.

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