Interface Type, Mode and Speed

The nature of the interface between the hard disk and the rest of the PC system plays a role in its overall performance. The type of interface is the sole determinant of the interface transfer performance specification, and the way the interface is actually implemented has an impact on the real-world performance of the storage subsystem.

The interface type of almost all drives today is either IDE/ATA or SCSI. Clearly, the decision of which to use involves a much "bigger picture" analysis of your needs and overall system design, since you can't interchange the drives. (I compare the two interfaces here.) However, even within each of the two types there are various modes that control how fast the interface runs. Choosing the best mode for your drive helps to ensure optimal performance. I discuss IDE/ATA modes here, and SCSI modes in this section.

While new hard disks are generally designed to be able to run at the fastest interface speeds possible for their interface, actually obtaining this interface speed requires an appropriate controller, hardware and/or drivers on the system side. Drives will generally "fall back" to slower interface speeds if required. For example, all new IDE/ATA drives are designed to run in Ultra DMA mode 5, allowing a theoretical maximum transfer rate of 100 MB/s. These drives will function on older interfaces at 66 MB/s, 33 MB/s or 16.7 MB/s if required. Performance will be negatively affected, but only significantly eroded if the speed of the interface falls below the maximum sustained transfer rate specification of the drive.

High interface speeds also require appropriate support from the system bus upon which the interface runs.

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