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 when the disk spins, each head traverses a circle, called a track, around the disk’s upper or lower surface. 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.

A disk storage system

Since a track can contain more information than we would normally want to manipulate at any 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 if bits (typical capacities are in the range of 512 bytes to a few KB), an 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 if 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 track 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 more sectors per track than the zone inside of it. In this manner, the storage space near the outer edge of the disk is used more efficiently than in a traditional disk system. 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 permanently 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 in a disk is damaged, the disk can be reformatted, although this process destroys all the information that was previously recorder or stored on the disk.

The capacity of a disk storage system depends on the number of a disks used and the density in which the tracks and sectors are placed. Lower-capacity systems consist of a single plastic disk known as a diskette or, in those cases in which the disk is flexible, by the less prestigious title floppy disk. Diskettes are easily inserted and removed form their corresponding read/write units and are easily stored. As a consequence, diskettes have been popular for off-line storage information. However, since the generic 3.5 inch diskettes have a capacity of only 1.44MB, their use has largely been replaced by other technologies.

High capacity disk systems, capable of holding many gigabytes, consist if perhaps five to ten rigid disk mounted on a common spindle. The fact that the disks used in these systems are rigid leads them to be known as hard-disk systems, in contrast to their floppy counterparts. To allow for faster 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 hard-disk systems are housed in cases that are sealed at the factory.

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 other)
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)
4. Transfer rate (the rate at which data can be transferred to or from the disk)

Hard-disk system generally have significantly better characteristics than floppy systems. Since the read/write heads do not touch the disk surface in a hard-disk system, one find rotation speeds of several thousand revolutions per-minute, whereas disks in floppy disk systems rotate on the order of 300 revolutions per-minute. Consequently, transfer rates for hard-disk systems, usually measured in MB per second, are much greater than those associated with floppy-disk systems, which tend to be measured in KB per second.

Since disk systems require physical motion for their operation, both hard and floppy systems suffer when compared to speed within electronic circuitry. Delay times within an electronic circuit are measured in units of nanoseconds or less, whereas seek times, latency times, and access times of a disk systems are measured in milliseconds. Thus the time required to retrieve information from a disk system can seem like an eternity to an electronic circuit awaiting 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. In these systems, information is recorded on the magnetic coating of a thin plastic tape that is wound on a reel of 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 a streaming tape units, which use tape similar in appearance to that in stereo systems to older, large reel-to-reel units. Although the capacity if those devices depends on the format used, most can hold many GB.

A major disadvantage of magnetic tape is that moving between different position on a tape can be very time-consuming owing to the significant amount of a 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 movement 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 application 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.

IBM Magnetic Tape Unit

Related entries:

1. Optical Systems
2. Flash Drives
3. Mass Storage
4. File Storage and Retrieval
5. Memory Organization