14.2 Choosing a Hard Disk

The good news about choosing a hard disk is that it's easy to choose a good one. Drive makers such as Maxtor and Seagate produce high-quality drives at similar price points for a given type and size drive. When you buy a hard disk in today's competitive market, you get what you pay for. That said, we will admit that we avoid IBM and Western Digital hard drives because we have experienced severe reliability problems with both makes.

Manufacturers often have two or more lines of drives that vary in several respects, all of which affect performance and price. Within a given grade of drive, however, drives from different manufacturers are usually closely comparable in features, performance, and price, if not necessarily in reliability. Neither is compatibility an issue, as it occasionally was in the early days of ATA. Any recent ATA hard disk coexists peacefully with any other recent ATA/ATAPI device, regardless of manufacturer. The same is generally true of SCSI drives. All of that said, we use Seagate and Maxtor ATA drives and Seagate SCSI drives when we have a choice.

Use the following guidelines when you choose a hard disk:

Choose the correct interface and standards

The most important consideration in choosing a hard disk is whether to use PATA, SATA, or SCSI, based on the issues we described in the preceding chapter. Once you make that decision, choose a drive that supports the proper standards. For more about ATA versus SCSI, see the upcoming sidebar.

PATA

Choose a PATA drive if you are building or upgrading a budget or mainstream PC that lacks SATA interfaces. Any drive you buy should support UDMA Mode 5 (ATA-100) or UDMA Mode 6 (ATA-133). Only Maxtor produces ATA-133 drives. ATA-100 has more bandwidth than even the fastest current drives require, so ATA-133 has no real performance advantage. Choose a drive in the size, performance, and price range you want and don't worry about ATA-100 versus ATA-133.

SATA

Choose an SATA drive if you are building or upgrading a budget or mainstream PC that has SATA interfaces, and if an SATA drive is available for about the same price as the comparable PATA model. We listed the benefits of SATA relative to PATA in the preceding chapter, and although those benefits are real, they are seldom worth paying much extra for. If your system has SATA interfaces and the SATA drive you want costs only $5 or $10 more than the PATA model, it's worth choosing SATA. But if the price differential is much larger, or if you would have to buy a separate SATA interface card to use the SATA drive, stick with PATA.

Many hard drives are available with either PATA or SATA interfaces, often with nearly identical model numbers. The only obvious differences may be the data and power connectors, shown in Figure 14-1, but more significant differences between models may exist. For example, the top drive is a Seagate ST3120023A Barracuda ATA V, with a 2 MB buffer and average seek time of 9.4 ms. The bottom drive is a Seagate ST3120023AS Barracuda Serial ATA V, which has an 8 MB buffer and average seek time of 9.0 ms. The model numbers differ by only one character and the names are similar, but the SATA model is a faster drive.

Figure 14-1. Two similar hard drives, with PATA (top) and SATA interfaces

SCSI

If disk performance is a major consideration, buy Ultra160 or Ultra320 SCSI drives. Even Ultra160 has sufficient bandwidth to support two 10,000 or 15,000 RPM SCSI drives, so Ultra320 provides no performance benefit for desktop systems. However, Ultra320 SCSI drives work properly on an Ultra160 interface and usually sell for little or no more than Ultra160 drives, so it makes sense to choose the faster interface. Purchase only SCAM-compliant drives.

It's tempting to buy the largest drive available, but that's not always the best decision. Very large drives often cost much more per gigabyte than mid-size drives, and the largest drives may have slower mechanisms than mid-size drives. So, in general, decide what performance level you need and are willing to pay for, and then buy a drive that meets those performance requirements, choosing the model based on its cost per gigabyte. All of that said, it may make sense to buy the largest drive available despite its high cost per gigabyte and slower performance, simply to conserve drive bays and ATA channels.

Choose the best rotation rate for your application

Rotation rate specifies how fast the drive spins. For years, all hard drives rotated at 3,600 RPM. Several years ago, drives that rotated at 5,400 or 7,200 RPM became available, initially for servers. This higher rotation speed has two benefits. First, a drive that rotates faster moves more data under the heads in a given amount of time, providing faster throughput. Second, the higher the rotation speed, the lower the latency.

Nowadays, 5,400 RPM ATA drives are used primarily in "appliance" applications such as set-top boxes and entry-level systems, where saving a few bucks in manufacturing cost is a major consideration. Some high-capacity ATA drives use 5,400 RPM mechanisms because these drives are typically used for secondary or "near-line" storage, for which lower performance is an acceptable trade-off for reduced cost. Mainstream ATA drives rotate at 7,200 RPM, and high-performance models at 10,000 RPM. Entry-level SCSI drives rotate at 7,200 RPM, mainstream models rotate at 10,000 RPM, and high-performance models at 15,000 RPM. All other things being equal, higher rotation speed provides faster data access and transfer rates, but with correspondingly higher noise and heat.

We recommend using 7,200 RPM or 10,000 RPM ATA and SCSI drives for mainstream applications. Choose a 5,400 RPM ATA model only when cost is an overriding concern, and even then you'll save only a few dollars by buying a 5,400 RPM drive rather than a 7,200 RPM unit. Choose a 15,000 RPM SCSI drive only if getting the highest possible disk performance outweighs the significant additional cost.

Give seek/access times heavy weight if you work mostly with many small files

Seek time is a measure of how quickly the head actuator can reposition the heads to a different track. Statistically, for a random access, the drive heads on average have to move across one-third of the disk surface. The time they require to do so is called the average seek time. Once the head arrives at the proper track, it must wait until the proper sector of that track arrives under the head before it can read or write data, which is called latency. Average latency is one-half the time that the disk requires to perform a full revolution. A 7,200 RPM drive, for example, turns at 120 revolutions per second and requires 8.33 milliseconds (ms) for each full revolution. The average latency is one-half of that, or 4.17 ms. The sum of the average seek time and average latency is called average access time, and is the best measure of a drive's access performance. Do not compare average seek time of one drive to average access time of another. Because average latency is a fixed value that is determined solely by the drive's rotation speed, you can easily convert back and forth between average seek time and average access time to make sure you're comparing apples to apples. For a 5,400 RPM drive, look for an average access time of 19 milliseconds (ms) or less; for a 7,200 RPM ATA or SCSI drive, 14 ms; for a 10,000 RPM drive, 8 ms; and for a 15,000 RPM drive, 6 ms.

Even within a model series, average seek times may differ significantly by drive capacity, and may also differ for reads versus writes. For example, seek times for the 18.4 GB Seagate Barracuda ES2 are 6.9 ms for reads and 7.5 ms for writes, while seek times for the 36.9 GB Barracuda ES2 are 8.5 ms for reads and 9.25 ms for writes. Smaller drives are often noticeably faster than larger models from the same series. Accordingly, when we build a system for which disk performance is paramount, we configure it with a small, fast primary drive to store the operating system, applications, and primary data, and a larger, slower secondary drive to store everything else. For example, we've built several systems with 15,000 RPM 18.4 GB Seagate Cheetahs as the primary drive and 7,200 RPM 100+ GB Seagate Barracudas (SCSI or ATA) as the secondary drive. That achieves the goal of fast disk performance at a reasonable price. Of course, on a system for which price is no object, we would simply use an array of the fastest 15K drives available.

Give data transfer rate heavy weight if you work mostly with large files

In most applications, data transfer rate (DTR) is less important to overall performance than average access time. DTR does become crucial if you work primarily with relatively few large files (sequential access) rather than many smaller files (random access). DTR is determined by several factors, the most important of which are disk rotation speed, cache size, and the onboard circuitry. When comparing advertised DTRs, be aware that there are several possible ways to list them, including internal versus external and burst versus sustained. The various transfer rates of drives are normally well-documented on the detailed specification sheets available on their web sites, and less well-documented in typical marketing materials.

Overall, the most important basis for comparison is the sustained transfer rate. Note that on drives that use more sectors on the larger outer tracks, transfer rates can vary significantly between inner and outer tracks. For example, a Seagate Cheetah 15K.3 drive has transfer rates of 49 MB/s on inner tracks and 75 MB/s on outer tracks. The average of those numbers, called the average formatted transfer rate, is a good yardstick. For an entry-level ATA drive, look for an average formatted transfer rate of 14 MB/s or higher; for a mainstream ATA drive, 30 MB/s or higher; for a 7200 RPM SCSI drive, 35 MB/s or higher; for a 10,000 RPM SCSI drive, 50 MB/s or higher; and for a 15,000 RPM SCSI drive, 55 MB/s or higher. Note that none of these transfer rates is fast enough to saturate ATA-100, let alone SATA or Ultra160 SCSI.

Rotation rate, average access time, and DTR are all favored by drives with smaller form factors, and in particular those with smaller platters and higher data densities. This is true because it is easier and less expensive to run small platters at high speed than large platters, and because the smaller physical size of the platters means that heads need not move as far to access data on any portion of the platter.

Get a model with large cache if it doesn't cost much more

Disk drives contain cache memory, which in theory provides benefits similar to those provided by L2 cache on a CPU. Entry-level and mainstream drives typically have 2 MB, and high-performance drives may have 8 MB or more. Some manufacturers sell the same model drive with differing amounts of cache, often indicated by a different letter on the end of the model number. In our experience, larger caches have a relatively small impact on overall drive performance, and are not worth paying much for. For example, given two otherwise identical drive models, one with 2 MB cache and one with 8 MB cache, we might pay $5 or $10 more for the 8 MB model, but not more. Adding cache is cheap, but it doesn't provide the benefits of a fast head mechanism and a fast rotation rate, both of which are more expensive to implement.

Make sure the drive fits your computer

All drives use standard width/height dimensions and screw hole positions to allow them to fit standard mounting locations. Drives for standard PCs are available in two nominal widths, named for the size of the platters they use. Each width is available in different heights. Together, the width and height describe the form factor of the drive, as follows:

5.25-inch

Some drives, typically of large capacity, use the 5.25-inch form factor. These drives actually measure 6 inches wide and come in three heights. Full-height devices measure 3.25 inches vertically, and are relatively uncommon nowadays. About the only 5.25-inch full-height drives you may encounter are very large capacity SCSI hard disks intended for use in servers. Half-height drives measure 1.625 inches vertically, and are far more common. A few 5.25-inch drives have been made in third-height form, which measure 1 inch vertically. Any of these drives fits in standard 5.25-inch drive bays. All cases except some low-profile cases have at least one full-height 5.25-inch drive bay, which can also be used instead to hold two half-height 5.25-inch drives.

Relative to 3.5-inch hard drives, 5.25-inch drives typically have slower rotational speed, longer seek times, and higher latency, all of which translate to slower DTRs. These performance drawbacks are true regardless of the capacity or interface of the drive. The one advantage of 5.25-inch drives is that their larger physical size allows packing in more and larger platters, which in turn means that 5.25-inch drives, particularly full-height models, can have much larger capacities than 3.5-inch drives. Although many 5.25-inch SCSI drives indeed have very high capacities, this is not the case with 5.25-inch ATA drives. Such drives, notably the Quantum Bigfoot series, are low-end drives that are commonly found in consumer-grade PCs. These drives gain no advantage from their larger form factor. One of the best upgrades you can make to a system is to replace one of these 3,600 or 4,000 RPM 5.25-inch ATA hard drives with a modern 3.5-inch 7,200 or 10,000 RPM drive.

3.5-inch

Most hard drives use the 3.5-inch form factor. These drives actually measure 4 inches wide and come in two heights. Most drives are third-height, or 1-inch high. Some high-capacity 3.5-inch hard drives use the 1.625-inch high half-height form factor.

Pay attention to how much current the drive draws

Here's one that few people think about, but that can be critical. A drive that requires only a few watts at idle or during read/write operations can easily require 30 watts or more when it spins up. Spinning up three or four ATA drives (or even one high-performance SCSI drive) may draw more current than your power supply can comfortably provide. Nearly all modern drives and BIOSs automatically support staged spin-up, whereby the Primary Master ATA drive (or Drive 0 on the SCSI chain) spins up first, with other devices spinning up only after enough time has passed to allow each earlier device to complete spin-up. However, not all drives and not all systems stage spin-up, so note the startup current requirements of a drive before you add it to a heavily loaded system. The current requirements of a drive are normally detailed in the technical specification sheets available on the drive manufacturer's web site.

Consider length of warranty

In preceding editions, we didn't even mention warranty. Nearly all drives, at least retail-boxed models, had warranties of two years or longer, sometimes much longer. Length of warranty was a nonissue because most drives either failed right out of the box or lasted until they were too small to be useful. We frequently didn't return drives that had failed after a couple of years and were still under warranty because a replacement drive of the same capacity would have been too small to bother installing.

Things changed almost overnight in late 2002 when, following close on the heels of reported widespread problems with some Fujitsu drive models, every major drive maker except Samsung reduced their standard warranties from three or five years to one year. Conspiracy theorists had a field day, speculating that drive makers were cost-reducing their drives and shortening their warranties in the expectation that newer drives would have greatly increased failure rates. We don't believe that for a second.

Hard drive factories cost billions, and no manufacturer is going to risk that investment by producing failure-prone drives. We think it's more likely that drive makers were forced by plummeting hard drive prices, shrinking margins, and the hideously bad high-tech economy to cut costs. The administrative and other costs involved in replacing one drive returned under warranty probably exceeds the profit from selling 10 or even 100 new drives. We think drive makers reduced warranties to a year to minimize the infrequent but very costly need to replace older drives. We suspect that current hard drives are at least as reliable as the older models that had longer warranties, and we do not hesitate to use and recommend drives that have only one-year warranties.

That said, if length of warranty is important to you, some manufacturers do offer "premium" lines at somewhat higher prices. In addition to their longer warranties, these models may have a larger cache, typically 8 MB rather than 2 MB. We might be tempted to pay a few extra bucks for a longer warranty and larger cache, but for most purposes we regard the standard warranty as acceptable.

Here are some things that you can safely ignore when shopping for a drive:

MTBF

Mean Time Between Failures (MTBF) is a technical measure of the expected reliability of a device. All modern ATA drives have extremely large MTBF ratings, often 50 years or more. That doesn't mean that the drive you buy will last 50 years. It does mean that any drive you buy will probably run for years (although some drives fail the day they are installed). The truth is that most hard drives nowadays are replaced not because they fail, but because they are no longer large enough. Ignore MTBF when you're shopping for a drive.

MTTR

Mean Time to Repair (MTTR) is another measure that has little application in the real world. MTTR specifies the average time required to repair a drive. Since nobody except companies that salvage data from dead drives actually repairs drives nowadays, you can ignore MTTR.

Shock rating

Drives are rated in gravities (G) for the level of shock they can withstand in both operating and nonoperating modes. For drives used in desktop systems, at least, you can ignore shock rating. All modern drives are remarkably resistant to damage if dropped, but all of them break if you drop them hard enough.