Picture, if you can, how many filing-cabinet drawers would be required to hold the millions of files of, say, tax records kept by the Internal Revenue Service or historical employee records kept by General Motors. The record storage rooms would have to be enormous. Computers, in contrast, permit storage on tape or disk in extremely compressed form. Storage capacity is unquestionably one of the most valuable assets of the computer.
Secondary storage, sometimes called auxiliary storage, is storage separate from the computer itself, where you can store software and data on a semi permanent basis. Secondary storage is necessary because memory, or primary storage, can be used only temporarily. If you are sharing your computer, you must yield memory to someone else after your program runs; if you are not sharing your computer, your programs and data will disappear from memory when you turn off the computer. However, you probably want to store the data you have used or the information you have derived from processing; that is why secondary storage is needed. Furthermore, memory is limited in size, whereas secondary storage media can store as much data as necessary. Keep in mind the characteristics of the memory hierarchy that were described in the section on the CPU and memory:
| Storage | Speed | Capacity | Relative Cost ($) | Permanent? |
| Registers | Fastest | Lowest | Highest | No |
| RAM | Very Fast | Low/Moderate | High | No |
| Floppy Disk | Very Slow | Low | Low | Yes |
| Hard Disk | Moderate | Very High | Very Low | Yes |
The benefits of secondary storage can be summarized as follows:
- Capacity. Organizations may store the equivalent of a roomful of data on sets of disks that take up less space than a breadbox. A simple diskette for a personal computer holds the equivalent of 500 printed pages, or one book. An optical disk can hold the equivalent of approximately 400 books.
- Reliability. Data in secondary storage is basically safe, since secondary storage is physically reliable. Also, it is more difficult for unscrupulous people to tamper with data on disk than data stored on paper in a file cabinet.
- Convenience. With the help of a computer, authorized people can locate and access data quickly.
- Cost. Together the three previous benefits indicate significant savings in storage costs. It is less expensive to store data on tape or disk (the principal means of secondary storage) than to buy and house filing cabinets. Data that is reliable and safe is less expensive to maintain than data subject to errors. But the greatest savings can be found in the speed and convenience of filing and retrieving data.
These benefits apply to all the various secondary storage devices but, as you will see, some devices are better than others. We begin with a look at the various storage media, including those used for personal computers, and then consider what it takes to get data organized and processed.
Diskettes and hard disks are magnetic media; that is, they are based on a technology of representing data as magnetized spots on the disk with a magnetized spot representing a 1 bit and the absence of such a spot representing a 0 bit.
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Diskettes
Made of flexible Mylar, a diskette can record data as magnetized spots on tracks on its surface. Diskettes became popular along with the personal computer.
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| Figure 1: Diskettes |
- Portability. Diskettes easily transport data from one computer to another. Workers, for example, carry their files from office computer to home computer and back on a diskette instead of in a briefcase. Students use the campus computers but keep their files on their own diskettes.
- Backup. It is convenient to place an extra copy of a hard disk file on a diskette.
- New software. Although, for convenience, software packages are kept on hard disk, new software out of the box comes on CD-ROM disks, which will be discussed further in the text.
For more details on floppy disks and drives click here. This is optional reading.
Hard Disks
A hard disk is a metal platter coated with
magnetic oxide that can be magnetized to represent data (Figure 2). Hard disks come in a
variety of sizes.
Hard disks for mainframes and minicomputers
may be as large as 14 inches in diameter. Several disks can be assembled into
a disk pack. There are different types of disk packs, with the number of
platters varying by model. Each disk in the pack has top and bottom surfaces
on which to record data. Many disk devices, however, do not record data on the
top of the top platter or on the bottom of the bottom platter.
Figure 2: Hard Disk and
Drive
A disk
drive is a machine that allows data to be read from a disk or written on a
disk. A disk pack is mounted on a disk drive that is a separate unit connected
to the computer. Large computers have dozens or ever hundreds of disk drives.
In a disk pack all disks rotate at the same time although only one disk is
being read or written on at any one time. The mechanism for reading or writing
data on a disk is an access arm; it moves a read/write head into position over
a particular track. The read/write head on the end of the access arm hovers
just above the track but does not actually touch the surface. When a
read/write head does accidentally touch the disk surface, this is called a
head crash and all data is destroyed. Data can also be destroyed if a
read/write head encounters even minuscule foreign matter on the disk surface.
A disk pack has a series of access arms that slip in between the disks in the
pack. Two read/write heads are on each arm, one facing up for the surface
above it and one facing down for the surface below it. However, only one
read/write head can operate at any one time.
In some disk drives the
access arms can be retracted; then the disk pack can be removed from the
drive. Most disk packs, however, combine the disks, access arms, and
read/write heads in a sealed module called a Winchester disk. Winchester disk
assemblies are put together in clean rooms so even microscopic dust particles
do not get on the disk surface.
Hard disks for personal computers are
5-1/4 inch or 3-1/2 inch disks in sealed modules and even gigabytes are not
unusual. Hard disk capacity for personal computers has soared in recent years;
capacities of hundreds of megabytes are common and gigabytes are not unusual.
Although an individual probably cannot imagine generating enough
output-letters, budgets, reports, and so forth-to fill a hard disk, software
packages take up a lot of space and can make a dent rather quickly.
Furthermore, graphics images and audio and video files require large file
capacities. Perhaps more important than capacity, however, is the convenience
of speed. Personal computer users find accessing files on a hard disk is
significantly faster and thus more convenient than accessing files on a
diskette.
For more details on hard disks and drives found in modern PCs, click here. This is
optional reading.
Removable Storage: Zip Disks
Personal computer users, who never seem to
have enough hard disk storage space, may turn to a removable hard disk
cartridge. Once full, a removable hard disk cartridge can be replaced with a
fresh one. In effect, a removable cartridge is as portable as a diskette, but
the disk cartridge holds much more data. Removable units also are important to
businesses concerned with security, because the units can be used during
business hours but hidden away during off hours. A disadvantage of a removable
hard disk is that it takes longer to access data than a built-in hard drive.
Figure 3: Iomega Zip
Disk
The most popular removable disk media is the Zip drive from Iomega (Figure 3). Over 100's of millions
have been sold, making it the de facto standard. The disk cartridges look like
a floppy disk, but are slightly bigger in all dimensions. Older Zip disks hold
100MB, newer ones hold up to 750MB and cost $4-$12 each, price increasing with capacity.
In comparison to Zip disks, Floppy disks hold 1.4MB and cost about 50 cents each.
The Zip drive sells for around $80- $125. Many new PCs come with
Zip drives built in addition to floppy drives. Zip disks are a great way to
store large files and software programs.
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 was 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.
Zone Recording
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 used to include upward of a dozen diskettes or more; all of which can now easily be placed 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 4.
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| Figure 4: Compact Disk (CD) and Drive) |
When buying a computer the speed of the CD-ROM drive is advertised using an "X" factor, like 12X, or 24X. This indicates the speed at which the CD can transfer data to the CPU - the higher the X factor, the faster the CD.
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.
DVDs
Digital Versatile Disk (DVD) drives are
now widely available in computers as well as home entertainment centers (Figure 5).
DVD-ROM drives can read data, such as stored commercial videos for playing.
DVD-RW allows for DVDs to be created on a computer.
The DVD is a flat disk, the size of a CD -
4.7 inches diameter and .05 inches thick. Data are stored in a small
indentation in a spiral track, just like in the CD. DVD disks are read by a
laser beam of shorter wavelength than used by the CD ROM drives. This allows
for smaller indentations and increased storage capacity. The data layer is
only half as thick as in the CD-ROM. This opens the possibility to write data
in two layers. The outer gold layer is semi transparent, to allow reading of
the underlying silver layer. The laser beam is set to two different
intensities, strongest for reading the underlying silver layer.
Figure 5: 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 MPEG decoder hardware (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; it is 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 audiotape, 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."