Why Two Disk Standards

Prior to ATA and SCSI, most microcomputer hard disks used the ST-506 standard, which required the computer to understand various disk-specific timing parameters, how the hard disk laid out data on its platters, and so on. Both ATA and SCSI created data abstractions in order to present a simpler and more uniform hard disk interface to the computer. They did this by shifting some of the burden of managing low-level disk details onto the drive itself.

SCSI is the more sophisticated of the two disk standards. Some of the features that differentiate SCSI and ATA include the following:

Devices Per Bus In the context of disk devices, a bus is a means of connecting disk devices to a computer via a single cable or daisy-chained set of cables. ATA supports just two devices per bus, with most ATA controllers providing two busses. SCSI supports up to seven or 15 devices, depending upon the SCSI variety. On most systems, each bus consumes one interrupt request (IRQ) line, and because the number of IRQs in an IA-32 computer is limited, this gives

SCSI a strong advantage for systems that need many disks. In practice, though, SCSI's advantage isn't as great as it might seem, because cable-length limits make it difficult to support more than half a dozen SCSI devices on most types of SCSI busses. Also, modern systems usually allow multiple devices to share a single IRQ, so the IRQ limitation isn't as severe as it once was.

Termination SCSI busses must be terminated by using special resistors. This requirement caused early SCSI systems to be tricky to configure when compared to ATA busses, which don't require special termination. More recent SCSI systems help automate termination, so this problem is less severe than it once was. Restoring complexity to SCSI, though, is the fact that different variants require different types of termination, which can be confusing and troublesome.

Multitasking The SCSI bus was designed to support multiple simultaneous transfers. For instance, if a specific type of SCSI bus is capable of 80MB/S transfers, and if two devices are connected to that bus, each of which is capable of 40MB/S transfers, then both devices can handle full-speed (40MB/s) transfers at once, saturating the SCSI bus's 80MB/s speed. Neither device is slowed down by the other's transfer. ATA, by contrast, is fundamentally single-tasking; in the same situation, only one device can transfer data at once, resulting in an overall system slowdown. Of course, because ATA supports only two devices per controller, this limitation isn't as severe for ATA as it would be for SCSI.

Tip To improve the performance of a two-disk system, put each disk on its own ATA bus. For instance, one bus might have the main hard disk and a CD-ROM drive, and the second bus might have a second disk and a tape backup drive. With this configuration, the system can access both disks simultaneously, improving performance in some multitasking situations.

Range of Devices Historically, more devices have been available for SCSI than for ATA. The original ATA specification was intended only for hard disks, whereas SCSI has long supported other devices. Through the 1990s, though, ATA has acquired standards for handling many nondisk device types. (ATA still doesn't support most external device types, such as scanners and printers.)

Speed and Capacity Historically, SCSI drives have been faster and available in higher capacities than ATA drives. These features haven't been so much a matter of the interfaces themselves as they've been ones of drive marketing; manufacturers have released their fastest and largest drives in SCSI form first. Today, this difference is smaller than it once was, and in fact it's easier to find ATA drives in very high capacities than to find similarly large SCSI drives.

Price SCSI devices have traditionally been more expensive than ATA devices of similar capacity. This price premium has increased in the last few years.

In the mid-1980s, SCSI found favor on Unix workstations and servers, and it was also Apple's favored disk type for its Macintosh computers. IA-32 systems, by contrast, generally used ATA drives. As IA-32 systems have gained prominence in the marketplace, ATA disks have gained a similarly dominant position. Today, even many Macintoshes and non-IA-32 workstations use ATA drives. You can still use SCSI devices, even on IA-32 computers, but this is definitely the less common approach.

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