Disk Drive Reliability

The disk drive marketplace is highly competitive because there is a huge consumer demand for disks with high capacities, fast transfer rates, and low costs. In order to meet these goals, disk manufacturers are continually advancing the state of the art for magnetic coatings, high-strength magnets (used in the motors that drive disk drive arms), and ultra-clean manufacturing technology.

In order to achieve high data transfer rates, a hard disk drive has to spin very fast: from 4,000 to 15,000 rpm (revolutions per minute). Also, magnetic patches that store data bits on the hard disk tend to be small and packed closely together. Therefore, in order to read these tiny little bits spinning at such high speed accurately, the read/write head has to be very, very close to the disk surface. The head rides on a cushion of air, and the distance between it and the disk surface is far less than the width of a human hair—less than the size of a single dust particle. In fact, if a head encounters a dust particle sitting on the surface of a disk while the disk is spinning at several thousand rpm, the head will crash into the disk, damaging itself and the magnetic coating on the disk. It is important to back up all data onto tape or another disk regularly.

A common specification for disk drive reliability is mean time between failures (MTBF), the average time (in hours) a device is expected to function before failing. Typically, disk drives for PCs have MTBF ratings of about 500,000 hours, 57 years. However, MTBF is a theoretical estimate because drives have not existed long enough for their life expectancy to be verified. The MTBF rating should be used in conjunction with service life. Service life is the amount of time before failures occur due to increased wear and tear of the component devices. It is how long the manufacturer predicts the disk would last. When shopping for disks, use the service life as a more accurate estimate for how long the disk will last before failures occur.

Optical Media: CDs versus DVDs

Data in an optical media is read and written using laser beams. Compact discs (CDs) and digital video discs (DVDs) are optical disks. A DVD is an enhanced form of a CD. The two types of disks are physically the same size, but they differ in format. DVDs offer much greater capacity, which they achieve in two ways. First, DVDs have narrower tracks, so they can squeeze more tracks onto the same size disk. Both CDs and DVDs are read using light from a laser. But, the CD laser is red, while the DVD laser is blue. Red light has a longer wavelength than a blue light. The blue laser is thus able to produce a smaller beam, allowing it to focus on the tinier tracks of the DVD. The second way that DVDs achieve increased capacity over CDs is by using multiple layers of tracks. The blue laser is not only narrower, but also more powerful. Its beams can penetrate the plastic and focus at different depths. DVDs that are dual-layered actually have two sets of tracks on one side of the disk, one beneath the other. The laser beam can be focused on either the upper or lower layer. This doubles the capacity of one side of a DVD disk. It is also possible to put tracks on both sides of a DVD. A double layer double side (DLDS) DVD drive uses double layers and can read double-sided disks, giving it four times the capacity of a single layer single side (SLSS) drive.

CDs have two recordable formats, CD-R and CD-RW. While the Parsons and Oja textbook lists recordable DVDs, there are multiple standard formats for this. So, while CD-Rs and CD-RWs are readable on most CD or DVD drives, recordable DVDs may not be readable on some types of DVD drives. For this reason, users who record data on optical media for widespread distribution (for example, for releasing a new software package) may wish to limit themselves to CDs for now even though it has lower capacity than DVDs. DVDs are usually used as a medium for multimedia presentations that combine sound with graphics, such as movies.