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Performance

Comparing the performance of the SCSI and IDE/ATA interfaces is not an easy task. While those who favor SCSI are quick to say that it is "higher performance" than IDE/ATA, this is not true all of the time. There are many different considerations and performance factors that interact when considering the performance equation, because performance is so dependent on system setup and on what is being done with the PC. I will try to look at some of these factors and how they influence system performance for both interfaces:

• Device Performance: When looking at particular devices, there is theoretically no difference between SCSI and IDE/ATA. The device itself should be the same in terms of its internal performance factors. In practice, this is rarely the case. Many manufacturers only make a particular drive as SCSI or IDE/ATA, not both, so direct comparisons aren't easy. Since SCSI is known to be the choice for those seeking performance, higher-performance drives tend to show up on the SCSI interface well before they do on IDE/ATA (you pay for this performance, of course, but that's a separate issue). Another issue is the implementation of the integrated device controller logic and the interface chip. Some companies that produce the same device (the physical hard disk assembly, for instance) for both SCSI and IDE/ATA may do a much better job of writing the control logic for one interface than for another. In general, SCSI drives offer higher performance than IDE/ATA ones.
• Maximum Interface Data Transfer Rate: The interface or external data transfer rate describes the amount of data that can be sent over the interface. As described here, it's important not to place too much emphasis on the interface transfer rate if you are only using a small number of devices. Comparing SCSI and IDE/ATA, both interfaces presently offer very high maximum interface rates, so this is not an issue for most PC users. However, if you are using many hard disks at once, for example in a RAID array, SCSI will offer better overall performance.
• Single vs. Multiple Devices and Single vs. Multitasking: For single devices, or single accesses (as in DOS), in many cases IDE/ATA is faster than SCSI, because the more intelligent SCSI interface has more overhead for sending commands and managing the channel. If you are just using a single hard disk, or doing simple work in DOS or Windows 3.x where everything happens sequentially, most of the benefits of SCSI are lost. For multitasking operating systems, especially where transfers are occurring between multiple devices, SCSI allows multitasking and command queuing and reordering, which enables devices to set up multiple transactions and have them take place basically simultaneously. In contrast, IDE/ATA transactions to one device "block" the channel and the other device cannot be accessed. Putting two devices on two different channels allows simultaneous access, but severely restricts expandability. IDE/ATA still does not have the advanced features that SCSI has for handling multiple devices.
• Device-Mixing Issues: IDE/ATA channels that mix hard disks and CD-ROMs are subject to significant performance hits in some situations, due to the fact that these are really different protocols on the same channel. SCSI does not have this problem.
• Technological Currency: IDE/ATA has one big advantage over SCSI in terms of performance, if cost is a consideration. Both interfaces are constantly being updated to offer faster performance, both in terms of the interfaces themselves and the drives produced for them. However, to take advantage of these improvements requires additional hardware purchases. For SCSI, the extra investments are much more costly than IDE/ATA. If one is on a limited budget, it could well be argued that staying current with IDE/ATA technology will offer better long-term performance than going with SCSI but only being able to upgrade every three or four years due to cost considerations.

Overall, SCSI is a higher-performance interface. For very simple applications, like a single hard disk and a single CD-ROM drive on different channels, IDE/ATA has a marginal advantage. For complex applications, SCSI has a significant advantage.

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