PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

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:

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.

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.

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

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.

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.

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.

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.

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.

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:

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.

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.

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.

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:

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.

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.

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.