SCSI Versus ATA - Advantages and Limitations

Modern operating systems are multitasking, and SCSI devices (with all their additional controller circuitry) function independently of one another, unlike ATA. Therefore, data can be read and written to any of the SCSI devices simultaneously. This enables smoother multitasking and increased overall data throughput.

The most advanced operating systems, such as Windows NT/2000/XP, even allow drive striping. A striped drive set is two or more drives that appear to the user as one drive. Data is split between the drives equally, again increasing overall throughput.

Increased fault tolerance and performance are readily implemented and supported in SCSI drive arrays. Ultra4 (Ultra320) SCSI drives offer advantages when compared with ATA. Ultra320 SCSI is 140% faster than UltraATA/133, which has a maximum data rate of 133MBps.

Ultra320 SCSI also fully supports multitasking and can significantly improve system performance in workstations and servers running Windows NT, 2000, or XP. ATA limits cable lengths to 18 inches, effectively eliminating the ability to connect remote or external devices, whereas Ultra4 (Ultra320) SCSI allows external connections of up to 12 meters or more in length.

Also note that ATA allows only 2 devices per cable, whereas Ultra320 SCSI can connect up to 15 devices. Finally, the domain validation feature of Ultra320 SCSI enables noise and other problems on the bus to be handled properly, whereas with ATA.

If a problem occurs with the connection (and that is more common at the UltraATA/100 and 133 speeds), the ATA drives simply fail. ATA drives have much less command overhead for a given sector transfer than do SCSI drives.

In addition to the drive-to-controller command overhead that both ATA and SCSI must perform, a SCSI transfer involves negotiating for the SCSI bus; selecting the target drive; requesting data; terminating the transfer over the bus; and finally converting the logical data addresses to the required cylinder, head, and sector addresses.

This arrangement gives ATA an advantage in sequential transfers handled by a single-tasking operating system. In a multitasking system that can take advantage of the extra intelligence of the SCSI bus, SCSI can have the performance advantage. SCSI drives offer significant architectural advantages over ATA and other drives.

Because each SCSI drive has its own embedded disk controller that can function independently from the system CPU, the computer can issue simultaneous commands to every drive in the system. Each drive can store these commands in a queue and then perform the commands simultaneously with other drives in the system.

The data could be fully buffered on the drive and transferred at high speed over the shared SCSI bus when a time slot was available. Although ATA drives also have their own controllers, they do not operate simultaneously, and command queuing is not supported.

In effect, the dual controllers in a dual-drive ATA installation work one at a time so as not to step on each other. ATA does not support overlapped, multitasked I/O, which enables a device to take on multiple commands and work on them independently and in an order different from which they were received, releasing the bus for other devices to use.

The ATA bus instead waits for each command to be completed before the next one can be sent. As you can see, SCSI has some advantages over ATA, especially where expansion is concerned, and also with regard to support for multitasking operating systems. Unfortunately, it also costs more to implement.

One final advantage of SCSI is in the portability of external devices. It is easy to take an external SCSI CD-ROM, tape drive, scanner, or even a hard disk and quickly move it to another system.

This allows moving peripherals more freely between systems and can be a bonus if you have several systems with which you might want to share a number of peripherals. Installing a new external SCSI device on a system is easier because you normally will not need to open it up.

Of course, moving an external SCSI device from one system to another is not nearly as easy as it would be with a USB or FireWire (IEEE-1394a) device, which is one reason the latter two (especially USB) are catching on in the mainstream market.

USB and IEEE-1394 (FireWire) devices are Plug and Play, allowing you to attach or remove the device with the power on. Plus, no device ID or other configuration is necessary. Narrow and Wide SCSI devices must be attached with the device and system power turned off; they also must be configured to use a unique device ID and have proper termination set for existing and new devices.