PC Hardware in a Nutshell, 2nd Edition

13.3 ATA Versus SCSI

Relative to ATA, SCSI has the following advantages:

Performance

ATA drives simply cannot compare to SCSI drives in performance under load. In our real-world testing, the slowest SCSI drives we used were faster than the fastest ATA drives, particularly under Windows NT/2000/XP, Linux, and other multitasking operating systems. This held true across the board, even when we tested an elderly, midrange Seagate SCSI drive against the fastest of the current ATA drives. Although ATA may match SCSI under light load, when disk activity starts to climb SCSI is simply faster. Don't let anyone convince you otherwise.

To verify our impression of SCSI versus ATA, we did an experiment. Barbara's main workstation uses a 7,200 RPM SCSI Seagate Barracuda drive. We built an identical system, but substituted a 7,200 RPM Seagate Barracuda ATA IV drive. For normal operations, performance of the two PCs was indistinguishable. We then started an XCOPY operation that streamed gigabytes of data comprising hundreds of directories and thousands of files from a third system across our 100BaseT network to the hard drive of the ATA system. While that data was being copied, the ATA system was nearly unusable. Loading Word from the hard drive took literally a full minute, and opening a large document took even longer. We then repeated the experiment, but this time to Barbara's SCSI Barracuda. The drive banged away, certainly, but we were able to load programs and run things normally with very little performance degradation. All this doesn't mean that the Seagate Barracuda ATA IV is a bad drive. It isn't. In fact, it's one of the best ATA drives available. But it does establish that ATA bogs down under load, whereas SCSI just keeps on ticking.

Bandwidth and concurrency

SCSI provides usable bandwidth at the nominal value stated. For example, a 160 MB/s Ultra160 SCSI channel in fact provides usable bandwidth of 160 MB/s, which may be shared among the devices on the channel. Given the actual 50 to 60 MB/s throughput of the fastest current hard disks, that means you can run three hard disks on an Ultra160 channel—all of which can read and/or write data simultaneously—without bandwidth becoming an issue. This is not true of ATA, because ATA allows only one device to use the channel at a time, regardless of how much bandwidth may be going unused. For example, if you connect two ATA-66 drives to an ATA-66 interface, and each drive has actual throughput of 25 MB/s, the data rate on that channel will never exceed 25 MB/s.

Reliability

In our experience, SCSI devices are simply more reliable than equivalent ATA devices, both in terms of the robustness of the devices themselves and the reliability of communication on the channel. For example, with some effort, an inexpensive ATAPI CD burner without BURN-Proof can usually be configured to run reliably without generating excessive coasters, whereas an equivalent SCSI burner simply works. We also believe that most SCSI devices are better-built than many ATA devices, although we have no hard evidence to prove this speculation.

Number of devices supported

A standard embedded dual-channel ATA interface supports at most four ATA/ATAPI devices, two per channel. A Narrow SCSI interface supports seven devices (besides the host adapter itself), and a Wide SCSI interface supports up to 15 devices. Many PCs now include a second hard disk, a tape drive, a CD burner, and so on. The ATAPI four-device limit may force trade-offs that you'd prefer not to make, such as removing the CD-ROM drive when you install a CD burner, or replacing a hard disk rather than adding a second hard disk. SCSI avoids this problem.

Resource demands

ATA uses system resources relatively inefficiently. An ATA interface requires one interrupt per two-device channel, whereas a SCSI host adapter supports as many as 15 devices on one bus, using only one interrupt. On older systems with PIO hard drives, the difference in CPU utilization can be immense. PIO- mode drives under load may demand 80% to 95% of the CPU, whereas SCSI drives (or ATA drives operating in DMA mode) may require from 0.5% to 2%.

Cable length and support for external devices

ATA is limited to 1.5 foot (0.46m) cables and officially supports only internal devices (although various workarounds are available that allow using ATA devices outside the main system enclosure). Depending on the version, SCSI supports cable lengths from about five feet (1.5m) to 39.4 feet (12m) or more. Adding external SCSI devices is no harder than installing internal ones—less so, actually, because you don't even need to open the PC case.

New technologies ship first in SCSI

Interface issues aside, the simple fact is that manufacturers treat ATA products as mass-market items, whereas their SCSI products are premium items. That means new technologies always arrive first in SCSI. For example, 7,200 RPM hard disks were available in SCSI long before the first 7,200 RPM ATA drive shipped. The fastest available ATA drives run at 7,200 RPM, whereas 7,200 RPM SCSI drives are entry-level, with 10,000 RPM drives the mid-range, and 15,000 RPM drives readily available. The same is true for such things as very fast head actuator mechanisms. They ship first in SCSI, sell at a premium there for a while, and then gradually make their way into mass-market ATA drives.

SCSI has the following disadvantages:

Cost

More than any other factor, the cost of SCSI has kept it from becoming a mainstream PC technology. A few years ago, the premium was outrageous. Nearly identical drives might sell for $400 with an ATA interface versus $1,200 with a SCSI interface. SCSI hard disks still sell at a premium over similar ATA drives, but that premium is now much smaller. For example, in June 2002, 20 GB 7200 RPM Seagate Barracuda ATA IV drives sold for about $90, while 7200 RPM 18.4 GB Seagate Barracuda Ultra160 SCSI drives sold for about $175. In addition to the higher drive cost, using SCSI also requires a SCSI host adapter, which costs $50 to $300.

Complex installation and configuration

ATA devices are simple to configure—set one jumper to specify Master or Slave, and connect the device to the primary or secondary ATA interface. Before the introduction of ATA-66 and ATA-100, which require a special 40-pin, 80-wire cable, ATA devices all used the same cables and connectors. Even with this change, installing and configuring ATA devices remains a straightforward task. SCSI, conversely, can be quite complex. The diversity of SCSI standards, cables, and connectors along with the need to specify SCSI IDs and to terminate the SCSI bus properly means there is more confusion and more room for errors when installing and configuring SCSI. For example, it is quite possible to buy a SCSI host adapter and a SCSI drive that are, if technically compatible, functionally mismatched. It is also possible to buy a SCSI cable that will not physically connect to the host adapter, the drive, or both. In practice, however, the widespread use of SCAM and the de facto standardization of cables and connectors have simplified installing and configuring SCSI to the level of ATA, at least for recent host adapters and devices.

Limited support by BIOSes and operating systems

Whereas ATA interface is rigidly defined and supported natively by all BIOSes and operating systems, SCSI remains an add-on technology. In practice, this is a smaller problem than it might seem, as SCSI host adapter manufacturers provide loadable supplemental BIOSes, ROM-based configuration and diagnostics utilities, and solid drivers for all common operating systems.

On balance, the determining factors are how heavily you use your hard drive, the operating system you use, and whether you can afford the additional cost of SCSI. Under heavy use, SCSI stands up to much higher loads without bogging down, and provides much snappier response. If you use your hard drive lightly and/or you run Windows 9X, SCSI may be overkill. But if you use your hard drive heavily and run Windows NT/2000/XP, get SCSI if you can afford it. Before you spend an extra $400 buying the fastest processor available, consider spending that extra money on SCSI instead. If most of your work is processor-bound, get the faster processor and ATA drives. But if much of your work is disk-bound, you'll find that the system with the slower processor and SCSI drives will provide better performance.

One of our friends, who'd just spent a bundle on a 2.2 GHz Pentium 4 system—the fastest available at the time—made the mistake of sitting down in front of one of our systems that has a 15,000 RPM Seagate Cheetah X15 SCSI drive installed. He fired up Word, turned to Robert, and asked if this was a dual-processor system. Robert told him it wasn't and asked what made him think it was. He said that everything just flew up onto the screen as soon as he double-clicked the icon, much faster than on his new system. When Robert told him that the system used a Celeron processor, the conversation became a bit strained. Robert finally stopped teasing him and explained that the system had an Adaptec Ultra160 SCSI host adapter and a 15,000 RPM Seagate Cheetah X15 drive. Kind of like one of those undercover police cars that looks like a junkyard reject but has a 500 HP engine.