Mass Storage Published by Ali Hassan - On 23:40:00 - This post haved 0 comments

Due to the volatility and limited size of a computer’s main memory, most computers have additional memory devices called mass storage (or secondary storage) systems, including magnetic disks, CDs, DVDs, magnetic tapes, and flash drives (all of which we will discuss shortly). The advantages of mass storage systems over main memory include less volatility, large storage capacities, low cost, and in many cases, the ability to remove the storage medium from the machine for archival purposes.
The terms on-line and off-line are often used to describe devices that can be either attached to or detached from a machine. On-line means that the device or information is connected and readily available to the machine without human intervention. Off-line means that human intervention is required before the device or information can be accessed by the machine—perhaps because the
device must be turned on, or the medium holding the information must be inserted into some mechanism.

A major disadvantage of mass storage systems is that they typically require mechanical motion and therefore require significantly more time to store and retrieve data than a machine’s main memory, where all activities are performed electronically.

Magnetic Systems

For years, magnetic technology has dominated the mass storage arena. The most common example in use today is the magnetic disk, in which a thin spinning disk with magnetic coating is used to hold data. Read/write heads are placed above and/or below the disk so that as the disk spins, each head traverses a circle, called a track. By repositioning the read/write heads, different concentric tracks can be accessed. In many cases, a disk storage system consists of several disks mounted on a common spindle, one on top of the other, with enough space for the read/write heads to slip between the platters. In such cases, the read/write heads move in unison. Each time the read/write heads are repositioned, a new set of tracks—which is called a cylinder—becomes accessible.

Since a track can contain more information than we would normally want to manipulate at any one time, each track is divided into small arcs called sectors on which information is recorded as a continuous string of bits. All sectors on a disk contain the same number of bits (typical capacities are in the range of 512 bytes to a few KB), and in the simplest disk storage systems each track contains the same number of sectors. Thus, the bits within a sector on a track near the outer edge of the disk are less compactly stored than those on the tracks near the center, since the outer tracks are longer than the inner ones. In fact, in high capacity disk storage systems, the tracks near the outer edge are

capable of containing significantly more sectors than those near the center, and this capability is often utilized by applying a technique called zoned-bit recording. Using zoned-bit recording, several adjacent tracks are collectively known as zones, with a typical disk containing approximately ten zones. All tracks within a zone have the same number of sectors, but each zone has more sectors per track than the zone inside of it. In this manner, efficient utilization of the entire disk surface is achieved. Regardless of the details, a disk storage system consists of many individual sectors, each of which can be accessed as an independent string of bits.

The location of tracks and sectors is not a permanent part of a disk’s physical structure. Instead, they are marked magnetically through a process called formatting (or initializing) the disk. This process is usually performed by the disk’s manufacturer, resulting in what are known as formatted disks. Most computer systems can also perform this task. Thus, if the format information on a disk is damaged, the disk can be reformatted, although this process destroys all the information that was previously recorded on the disk. The capacity of a disk storage system depends on the number of platters used and the density in which the tracks and sectors are placed. Lower-capacity systems may consist of a single platter. High-capacity disk systems, capable of holding many gigabytes, or even terabytes, consist of perhaps three to six platters mounted on a common spindle. Furthermore, data may be stored on both the upper and lower surfaces of each platter.

Several measurements are used to evaluate a disk system’s performance: (1) seek time (the time required to move the read/write heads from one track to another); (2) rotation delay or latency time (half the time required for the disk to make a complete rotation, which is the average amount of time required for the desired data to rotate around to the read/write head once the head has been positioned over the desired track); (3) access time (the sum of seek time and rotation delay); and (4) transfer rate (the rate at which data can be transferred to or from the disk). (Note that in the case of zone-bit recording, the amount of data passing a read/write head in a single disk rotation is greater for tracks in an outer zone than for an inner zone, and therefore the data transfer rate varies depending on the portion of the disk being used.) A factor limiting the access time and transfer rate is the speed at which a disk system rotates. To facilitate fast rotation speeds, the read/write heads in these systems do not touch the disk but instead “float” just off the surface. The spacing is so close that even a single particle of dust could become jammed between the head and disk surface, destroying both (a phenomenon known as a head crash). Thus, disk systems are typically housed in cases that are sealed at the factory. With this construction, disk systems are able to rotate at speeds of several thousands times per second, achieving transfer rates that are measured in MB per second.

Since disk systems require physical motion for their operation, these systems suffer when compared to speeds within electronic circuitry. Delay times within an electronic circuit are measured in units of nanoseconds (billionths of a second) or less, whereas seek times, latency times, and access times of disk systems are measured in milliseconds (thousandths of a second). Thus the time required to retrieve information from a disk system can seem like an eternity to an electronic circuit awaiting a result.

Disk storage systems are not the only mass storage devices that apply magnetic technology. An older form of mass storage using magnetic technology is magnetic tape (Figure 1.10). In these systems, information is recorded on the magnetic coating of a thin plastic tape that is wound on a reel for storage. To access the data, the tape is mounted in a device called a tape drive that typically can read, write, and rewind the tape under control of the computer. Tape drives range in size from small cartridge units, called streaming tape units, which use tape similar in appearance to that in stereo systems to older, large reel-to-reel units. Although the capacity of these devices depends on the format used, most can hold many GB.

A major disadvantage of magnetic tape is that moving between different positions on a tape can be very time-consuming owing to the significant amount of tape that must be moved between the reels. Thus tape systems have much longer data access times than magnetic disk systems in which different sectors can be accessed by short movements of the read/write head. In turn, tape systems are not popular for on-line data storage. Instead, magnetic tape technology is reserved for off-line archival data storage applications where its high capacity, reliability, and cost efficiency are beneficial, although advances in alternatives, such as DVDs and flash drives, are rapidly challenging this last vestige of magnetic tape.