Magnetic Disk and Flash Storage

10.2 Magnetic Disk and Flash Storage

Magnetic disks provide the bulk of secondary storage for modern computer systems. Although disk capacities have been growing year after year, the storage requirements of large applications have also been growing very fast, in some cases even faster than the growth rate of disk capacities. A very large database may require hundreds of disks. In recent years, flash-memory storage sizes have grown rapidly, and flash storage is increasingly becoming a competitor to magnetic disk storage for several applications.

10.2.1 Physical Characteristics of Disks

Physically, disks are relatively simple (Figure 10.2). Each disk platter has a flat, circular shape. Its two surfaces are covered with a magnetic material, and infor- mation is recorded on the surfaces. Platters are made from rigid metal or glass.

When the disk is in use, a drive motor spins it at a constant high speed (usually 60, 90, or 120 revolutions per second, but disks running at 250 revolutions per second are available). There is a read–write head positioned just above the surface of the platter. The disk surface is logically divided into tracks , which are subdivided into sectors .A sector is the smallest unit of information that can

be read from or written to the disk. In currently available disks, sector sizes are

10.2 Magnetic Disk and Flash Storage 433

track t

spindle

arm assembly sector s

cylinder c

read–write

head

platter

arm rotation

Figure 10.2 Moving head disk mechanism.

typically 512 bytes; there are about 50,000 to 100,000 tracks per platter, and 1 to

5 platters per disk. The inner tracks (closer to the spindle) are of smaller length, and in current-generation disks, the outer tracks contain more sectors than the inner tracks; typical numbers are around 500 to 1000 sectors per track in the inner tracks, and around 1000 to 2000 sectors per track in the outer tracks. The numbers vary among different models; higher-capacity models usually have more sectors per track and more tracks on each platter.

The read –write head stores information on a sector magnetically as reversals of the direction of magnetization of the magnetic material. Each side of a platter of a disk has a read–write head that moves across the platter to access different tracks. A disk typically contains many platters, and the

read–write heads of all the tracks are mounted on a single assembly called a disk

arm , and move together. The disk platters mounted on a spindle and the heads mounted on a disk arm are together known as head –disk assemblies . Since the heads on all the platters move together, when the head on one platter is on the ith track, the heads on all other platters are also on the ith track of their respective platters. Hence, the ith tracks of all the platters together are called the ith cylinder .

Today, disks with a platter diameter of 3 1 2 inches dominate the market. They have a lower cost and faster seek times (due to smaller seek distances) than do the larger-diameter disks (up to 14 inches) that were common earlier, yet they provide high storage capacity. Disks with even smaller diameters are used in portable devices such as laptop computers, and some handheld computers and portable music players.

The read–write heads are kept as close as possible to the disk surface to increase the recording density. The head typically floats or flies only microns

434 Chapter 10 Storage and File Structure

from the disk surface; the spinning of the disk creates a small breeze, and the head assembly is shaped so that the breeze keeps the head floating just above the disk surface. Because the head floats so close to the surface, platters must be machined carefully to be flat.

Head crashes can be a problem. If the head contacts the disk surface, the head can scrape the recording medium off the disk, destroying the data that had been there. In older-generation disks, the head touching the surface caused the removed medium to become airborne and to come between the other heads and their platters, causing more crashes; a head crash could thus result in failure of the entire disk. Current-generation disk drives use a thin film of magnetic metal as recording medium. They are much less susceptible to failure by head crashes than the older oxide-coated disks.

A disk controller interfaces between the computer system and the actual hardware of the disk drive; in modern disk systems, the disk controller is im- plemented within the disk drive unit. A disk controller accepts high-level com- mands to read or write a sector, and initiates actions, such as moving the disk arm to the right track and actually reading or writing the data. Disk controllers also attach checksums to each sector that is written; the checksum is computed from the data written to the sector. When the sector is read back, the controller computes the checksum again from the retrieved data and compares it with the stored checksum; if the data are corrupted, with a high probability the newly computed checksum will not match the stored checksum. If such an error occurs, the controller will retry the read several times; if the error continues to occur, the controller will signal a read failure.