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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Hard Disk Performance | Hard Disk External Performance Factors | Disk Interface Factors ]

Command Overhead and Multiple Device Considerations

As discussed in this section, a certain amount of overhead is required to process any command to the hard disk. However, that's only one type of overhead, the kind within the hard disk involved in doing a random access to the platters. There are other overhead considerations as well that exist within the system itself. These include the time for the system to process the command at a high level, operating system overhead, and so on. Every "piece" of this overhead reduces overall performance by a small amount.

In comparing the SCSI and IDE/ATA interfaces, command overhead is an important consideration. SCSI is a much more intelligent and capable interface, 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.

There is also another consideration: the number of devices that are sharing the interface. This is particularly a concern with SCSI, which allows for many devices on a bus (IDE/ATA and enhancements allow just two per channel). If you are using four hard disks on a SCSI bus in a server that is handling many simultaneous requests, and each drive has an internal sustained transfer rate of 18 MB/s, that 80 MB/s for Ultra2 Wide SCSI will probably, at many points in time, be in full use. On an IDE/ATA machine only one device can use any given channel at a time, so you only need to compare the speed of the interface to the speed of each drive that will use it, not the sum of their transfer rates.

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