|Storage||Speed||Capacity||Relative Cost ($)||Permanent?|
|Floppy Disk||Very Slow||Low||Low||Yes|
|Hard Disk||Moderate||Very High||Very Low||Yes|
|Figure 1: Diskettes|
For more details on floppy disks and drives click here. This is optional reading .
A hard disk is a metal platter coated with magnetic oxide that can be magnetized to represent data. Hard disks come in a variety of sizes.
|Figure 2: Hard Disk and Drive|
For more details on hard disks and drives found in modern PCs, click here. This is optional reading .
Removable Storage: Zip Disks
|Figure 3: Iomega Zip Disk|
For more details on
removable storage such as Zip drives, click here . This is optional reading .
Hard Disks in Groups
A concept of using several small disks that work together as a unit is called a redundant array of inexpensive disks, or simply RAID. The group of connected disks operates as if it were just one large disk, but it speeds up reading and writing by having multiple access paths. The data file for, say, aircraft factory tools, may be spread across several disks; thus, if the computer is used to look up tools for several workers, the computer need not read the data in turn but instead read them at the same time in parallel. Furthermore, data security is improved because if a disk fails, the disk system can reconstruct data on an extra disk; thus, computer operations can continue uninterrupted. This is significant data insurance.
How Data Is Organized on a Disk
There is more than one way of physically organizing data on a disk. The methods we will consider here are the sector method and the cylinder method.
The Sector Method
In the sector method each track is divided into sectors that hold a specific number of characters. Data on the track is accessed by referring to the surface number, track number, and sector number where the data is stored. The sector method is used for diskettes as well as disk packs.
The fact that a disk is circular presents a problem: The distances around the tracks on the outside of the disk are greater than that of the tracks or the inside. A given amount of data that takes up 1 inch of a track on the inside of a disk might be spread over several inches on a track near the outside of a disk. This means that the tracks on the outside are not storing data as efficiently.
Zone recording involves dividing a disk into zones to take advantage of the storage available on all tracks, by assigning more sectors to tracks in outer zones than to those in inner zones. Since each sector on the disk holds the same amount of data, more sectors mean more data storage than if all tracks had the same number of sectors.
The Cylinder Method
A way to organize data on a disk pack is the cylinder method. The organization in this case is vertical. The purpose is to reduce the time it takes to move the access arms of a disk pack into position. Once the access arms are in position, they are in the same vertical position on all disk surfaces.
To appreciate this, suppose you had an empty disk pack on which you wished to record data. You might be tempted to record the data horizontally-to start with the first surface, fill track 000, then fill track 001, track 002, and so on, and then move to the second surface and again fill tracks 000, 001, 002, and so forth. Each new track and new surface, however, would require movement of the access arms, a relatively slow mechanical process.
Recording the data vertically, on the other hand, substantially reduces access arm movement. The data is recorded on the tracks that can be accessed by one positioning of the access arms-that is, on one cylinder. To visualize cylinder organization, pretend a cylindrically shaped item, such as a tin can, were figuratively dropped straight down through all the disks in the disk pack. All the tracks thus encountered, in the same position on each disk surface, comprise a cylinder. The cylinder method, then, means all tracks of a certain cylinder on a disk pack are lined up one beneath the other, and all the vertical tracks of one cylinder are accessible by the read/write heads with one positioning of the access arms mechanism. Tracks within a cylinder are numbered according to this vertical perspective: A 20-surface disk pack contains cylinder tracks numbered 0 through 19, top to bottom.
The explosive growth in storage needs has driven the computer industry to provide cheaper, more compact, and more versatile storage devices with greater capacity. This demanding shopping list is a description of the optical disk, like a CD. The technology works like this: A laser hits a layer of metallic material spread over the surface of a disk. When data is being entered, heat from the laser produces tiny spots on the disk surface. To read the data, the laser scans the disk, and a lens picks up different light reflections from the various spots.
Optical storage technology is categorized according to its read/write capability. Read-only media are recorded on by the manufacturer and can be read from but not written to by the user. Such a disk cannot, obviously, be used for your files, but manufacturers can use it to supply software. Applications software packages sometimes include a dozen diskettes or more; all these could fit on one optical disk with plenty of room to spare. The most prominent optical technology is the CD-ROM, for compact disk read-only memory. The disk in its drive is shown in Figure 3.
|Figure 3: Compact Disk (CD) and Drive)|
Modern computers now offer a write CD drive or, CD-RW as an option. CD-RW is a write-once, read-many media. With a CD-RW drive, you can create your own CDs. This offers an inexpensive, convenient, safe way to store large volumes of data such as favorite songs, photographs, etc.
For more details on CD technology, click here. This is optional reading .
Digital Versatile Disk (DVD) drives are now widely available in computers as well as home entertainment centers. DVD-ROM drives can read data, such as stored commercial videos for playing. DVD-RW allow DVDs to be created on a computer.
|Figure 4: DVD Disk and Drive|
A 4.7 GB side of a DVD can hold 135 minutes top quality video with 6 track stereo. This requires a transmission rate of 4692 bits per second. The 17 GB disk holds 200 hours top quality music recording.
DVD movies are made in two "codes." Region one is USA and Canada, while Europe and Asia is region two. When you play movies, your hardware (MPEG decoder. MGEG is the data coding for movies similar to JPEG for pictures.) must match the DVD region. The movies are made in two formats, each with their own coding.
The DVD drives come in 2X, 4X, etc. versions, like the CD-ROM's.
The DVD drives will not replace the magnetic hard disks. The hard disks are being improved as rapidly as DVD, and they definitely offer the fastest seek time and transmission rate (currently 5-10 MB/second). No optic media can keep up with this. But the DVD will undoubtedly gain a place as the successor to the CD ROM and is playing an important role in the blending of computers and entertainment centers.
For more detail on DVD technology, click here . This is optional reading .
We saved magnetic tape storage for last because it has taken a subordinate role in storage technology. Magnetic tape looks like the tape used in music cassettes plastic tape with a magnetic coating. As in other magnetic media, data is stored as extremely small magnetic spots. Tapes come in a number of forms, including l/2-inch-wide tape wound on a reel, l/4-inch- wide tape in data cartridges and cassettes, and tapes that look like ordinary music cassettes but are designed to store data instead of music. The amount of data on a tape is expressed in terms of density, which is the number of characters per inch (cpi) or bytes per inch (bpi) that can be stored on the tape.
The highest-capacity tape is the digital audio tape, or DAT, which uses a different method of recording data. Using a method called helical scan recording, DAT wraps around a rotating read/write head that spins vertically as it moves. This places the data in diagonal bands that run across the tape rather than down its length. This method produces high density and faster access to data.
Two reels are used, a supply reel and a take-up reel. The supply reel, which has the tape with data on it or on which data will be recorded, is the reel that is changed. The take-up reel always stays with the magnetic tape unit. Many cartridges and cassettes have the supply and take-up reels built into the same case.
Tape now has a limited role because disk has proved the superior storage medium. Disk data is quite reliable, especially within a sealed module. Furthermore, as we will see, disk data can be accessed directly, as opposed to data on tape, which can be accessed only by passing by all the data ahead of it on the tape. Consequently, the primary role of tape today is as an inexpensive backup medium.
Although a hard disk is an extremely reliable device, a hard disk drive is subject to electromechanical failures that cause loss of data. Furthermore, data files, particularly those accessed by several users, are subject to errors introduced by users. There is also the possibility of errors introduced by software. With any method of data storage, a backup system a way of storing data in more than one place to protect it from damage and errors is vital. As we have already noted, magnetic tape is used primarily for backup purposes. For personal computer users, an easy and inexpensive way to back up a hard disk file is to simply copy it to a diskette whenever it is updated. But this is not practical for a system with many files or many users.
Personal computer users have the option of purchasing their own tape backup system, to be used on a regular basis for copying all data from hard disk to a high-capacity tape. Data thus saved can be restored to the hard disk later if needed. A key advantage of a tape backup system is that it can copy the entire hard disk in minutes, saving you the trouble of swapping diskettes in and out of the machine.
A rule of thumb among computer professionals is to estimate disk needs generously and then double that amount. But estimating future needs is rarely easy. Many users, therefore, make later adjustments like adding a removable hard disk cartridge to accommodate expanding storage needs. To quote many a computer user, "I just couldn't envision how I could use all that disk space. Now I can imagine even the extra disk filling up."