The second major statistic of importance in comparing hard disks is the disk transfer rates— how quickly the disk can transfer data to and from the computer. Note that I referred to disk transfer rates (plural). There are two reasons for this:
• The disk spins at the same rate no matter what cylinder is being read, but modern hard drive designs place more sectors along outer cylinders than along inner ones. This means that data read from the outer cylinders transfer faster than do data read from inner cylinders. Hence, the internal data transfer rate is variable.
• The data transfer rate from the platters is potentially different from the data transfer rate between the hard disk and the computer. The latter rate is determined by the computer's interface type, such as 66MBps for an Ultra-66 interface. The former is determined by data density on the platter and the speed with which the platter spins.
Hard disk manufacturers generally try to emphasize the external transfer rate—the speed of the hard disk's interface. This transfer rate, however, is far less important than the internal transfer rate—that rate of information transfer from the disk platters to the drive electronics.
To further complicate matters, hard disk manufacturers often quote internal data transfer rates in megabits per second (Mbps), but external transfer rates in megabytes per second (MBps). The abbreviations for these terms vary only in the case of the letter b, but the difference is a factor-of-eight value. When the internal rate is reported as 240Mbps and the external rate is 66MBps, it's easy to become confused and believe that the external rate is the bottleneck. It's not—expressed in MBps, the internal rate is only 30MBps.
Early hard disks used a fixed number of sectors per cylinder, no matter where on the disk that cylinder was located—an inner cylinder or outer one. This design resulted in a lower density of data on outer tracks than on inner ones. In the quest for increased disk storage capacity, engineers began squeezing more sectors onto outer cylinders than on inner ones, resulting in higher data capacity per platter (see Figure 5.5).
Fixed Sectors/Cylinder Variable Sectors/Cylinder
16 Sectors Total 24 Sectors Total
Actual hard disks use far more sectors and cylinders than are depicted here, but the basic principle still holds.
Floppy disks still used a fixed number of sectors per cylinder. Some exotic disk formats, such as the 400KB and 800KB formats used by early Macintoshes, varied the number of sectors per cylinder, but x86 floppies have never used such schemes.
In absolute terms, the number of bytes stored on each disk platter has increased substantially over the years. Hard drives sold in 2000 store tens of gigabytes in two or three 3.5-inch plat- h ters, as compared to a tenth the capacity on the same number of platters in the mid-1990s. This R
increase in data density translates into an increase in disk transfer speed, even when the o
rotational speed stays the same. When more data pass under the read/write head in any given period of time, more data are transferred to or from the hard disk.
The faster a hard disk spins, the faster it can transfer data. For this reason, disk spin rates have crept up over the years, from 3,600 rpm in the early 1990s to a minimum common speed of 5,400 rpm today. Indeed, 10,000 rpm hard disks are gaining in popularity today. All other things being equal, a faster hard disk is preferable to a slower one; however, all other things might not be equal. For instance, two 30GB hard disks might achieve their capacities in different ways. One might use three platters and spin at 10,000 rpm, whereas another might use two platters and spin at 7,200 rpm. With the greater data density on the latter drive, its 7,200 speed might actually result in faster data transfers than could be achieved from the 10,000 rpm drive. Another factor is the distribution of data in sectors as compared to in cylinders. A drive with fewer cylinders but more sectors per cylinder will be faster than one with more cylinders and fewer sectors per cylinder, all other things being equal.
In the end, to determine a disk's data transfer speed, you must rely upon transfer statistics provided by the manufacturer, or on test results printed in magazines or online. (Most such tests are conducted under Windows, but if you can find raw data transfer rates, those should not be much different than what you can obtain under Linux.)
As hard drives have spun faster, they've developed increasing problems with heat. The faster spin rates produce increased waste heat from friction and from the increased demands placed on electronic circuits that process the data. Not all drives that spin at the same rate generate the same amount of heat, however. I've owned 5,400 rpm drives that generate more heat than some 7,200 rpm drives. Nonetheless, as a general rule of thumb, you should at least consider extra heat dissipation measures for drives that spin at 7,200 rpm or faster. A 10,000 rpm drive almost always requires such special cooling.
The most common method of providing extra cooling to extra-hot drives is to add a hard drive cooling fan to your system. These devices mount in a 5.25-inch drive bay and allow you to mount a 3.5-inch drive within adapter rails. The front bezel incorporates two or three small fans that blow air onto the hard disk, thus keeping it cool. If you use a SCSI hard disk, you might want to consider mounting it in an external case with its own independent cooling system. This practice can help isolate the heat generated by the hard disk from other components of the computer, and isolate the hard disk from heat produced by other disks.
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