Computers:Tools for an Information Age

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Secondary Storage

Separate from the computer itself

Software and data stored on a semipermanent basis Unlike memory, not

lost when power is lost

Benefits

• Space• Reliability• Convenience• Economy

Benefits of Secondary Storage

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Space Store a roomful of data on disks smaller than the size of a breadbox

Diskette contains equivalent of 500 printed pages

Optical disk can hold equivalent of 500 books

Reliability Data in secondary storage is relatively safe

Secondary storage is highly reliable

More difficult for untrained people to tamper with data stored on disk

Convenience

Authorized users can easily and quickly locate data stored on the computer

Economy Several factors create significant savings in storage costs

Less expensive to store data on disks than to buy and house filing cabinets

Reliable and safe data is less expensive to maintain

Greater speed and convenience in filing and retrieving data

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Magnetic Disk Storage

Data represented as magnetized spots on surface of spinning disk Spots on disk

converted to electrical impulses

Primary types Diskettes- now

replaced with USB drives

Hard Disks

Hard Disks Rigid platter coated with

magnetic oxide Several can be combined

into a disk pack Disk drive - a device that

allows data to be read from or written to a disk

Disk drive for personal computers contained within computer housing

Large computer systems may have several external disk drives

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Reading/Writing Data

Access arm moves read/write head over particular location

Read/write head hovers a few millionths of an inch above platter If head touches platter,

a head crash occurs and data is destroyed

Data can be destroyed if head touches miniscule foreign matter on surface of disk

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Disk Packs

Each platter has its own access arm with read/write head

Most disk packs combine platters, access arms, and read/write head

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Hard Disks for Personal Computers

Sealed modules that mount in a 3 ½” bay

Capacity in gigabytes Accessing files much

faster than accessing files on diskettes

Some contain removable cartridges Iomega’s Jaz drive is

very popular

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Redundant Array Of Independent Disks (RAID)

A group of disks that work together as one Raid level 0 spreads data

from a single file over several drives

Called data striping Increases performance

Raid level 1 duplicates data on several drives

Called disk mirroring Increases fault tolerance

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How Data Is Organized

Track Sector Cluster Cylinder

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Track

The circular portion of the disk surface that passes under the read/write head Floppy diskette has 80 tracks on each

surface Hard disk may have 1,000 or more tracks

on each surface of each platter

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Sector

Each track is divided into sectors that hold a fixed number of bytes Typically 512 bytes per

sector Zone recording assigns

more sectors to tracks in outer zones than those in inner zones Uses storage space

more fully

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Cluster

A fixed number of adjacent sectors that are treated as a unit of storage Typically two to eight sectors, depending

on the operating system

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Cylinder

The track on each surface that is beneath the read/write head at a given position of the read/write heads When file is larger than the

capacity of a single track, operating system will store it in tracks within the same cylinder

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Disk Access Speed

Access time - the time needed to access data on disk

Three factors Seek time Head switching Rotational delay

Once data found, next step is data transfer

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Seek Time

The time it takes the access arm to get into position over a particular track All access arms move as a unit All simultaneously in position over a set of

tracks that make up a cylinder

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Head Switching

The activation of a particular read/write head over a particular track All access arms move together, but only

one read/write head can operate at any one time

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Rotational Delay

The time it takes for the desired data on the track to rotate underneath the read/write head On average, half the time for a complete

revolution of the disk

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Data Transfer

The process of transferring data between its location on the disk track and memory

Measures of performance Average access time

About 10 milliseconds (10 thousands of a second)

Can be improved by disk caching Data transfer rate - how fast data can be

transferred once it has been found Stated in terms of megabytes per second

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Disk Caching

Disk cache - a special area of memory When disk drive reads data from disk, it

reads adjacent data and stores it in memory

When next read instruction is issued, drive checks first to see if desired data is in disk cache

Similar to memory caching discussed in Chapter 4

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Optical Disk Storage

Provides inexpensive and compact storage with greater capacity

Laser scans disk and picks up light reflections from disk surface

Categorized by read/write capability Read-only media - user can read from, but not write

to disk Write-once, read-many (WORM) - user can write to

disk once Magneto-optical - combines magnetic and optical

capabilities

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Compact Disks

CD-ROM - drive can only read data from CDs CD-ROM stores up to 700 MB per disk Primary medium for software distribution

CD-R - drive can write to disk once Disk can be read by CD-ROM or CD-R drive

CD-RW - drive can erase and record over data multiple times Some compatibility problems trying to read CD-

RW disks on CD-ROM drives

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Digital Versatile Disk (DVD)

Short wavelength laser can read densely packed spots DVD drive can read CD-ROMs Capacity up to 17GB Allows for full-length movies Sound is better than on audio CDs

Several versions of writable and rewritable DVDs exist

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Multimedia

Presents information with text, illustrations, photos, narration, music, animation, and film clips

Not practical until the advent of the optical disk

Requirements Applications

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Requirements

CD-ROM or DVD-ROM drive Sound card or sound chip Speakers

For high-quality sound, get good speakers and powered subwoofer

Equipped to handle MPEG Standards for compressing video

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Applications

Education Go on virtual tours Study musical scores Study a foreign language

Other Prepare taxes with video clips from IRS

experts Play games

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Magnetic Tape Storage

Tape similar to tape used in music cassettes

Categorized in terms of density Number of bits per inch

stored on tape Used primarily for backup

of data stored on disk systems

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Backup Systems

Imperative to have copies of important data stored away from the computer Disks occasionally fail Software installation can cause computer to crash Users make mistakes entering data

Tape is ideal backup medium Can copy entire hard disk to single tape in

minutes Backup can be scheduled when you are not going

to use the system

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Organizing and Accessing Stored Data

Character Field Record File Database

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Character

A letter, digit, or special character

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Field

A set of related characters Describes one characteristic of a

person, place, or thing For a university, a student’s first name

would be stored in a field Key field - a unique identifier for a

record

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Record

A collection of related fields For the university, all of the fields for one

student constitute one record

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File

A collection of related records For university, all the student records

compose a file

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Database

A collection of related files stored with minimum redundancy (duplication) For university, student file, alumni file,

faculty/staff file, courses file, financial file, etc. would make up a database

Organized to make retrieving data easier

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File Plan Overview

Must devise a plan for placing data on a storage unit

Key factors Whether users must access data directly

(immediately) How data must be organized on disk Type of processing that will take place

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File Organization

Three major methods of organizing data files in secondary storage Sequential Direct Indexed

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Sequential File Organization

Records are stored in order according to a key field If a particular record is desired, all prior

records must be read first To update a record, a new sequential file

must be created, with changed and unchanged records

Tape storage uses sequential organization

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Direct File Organization

Also called random access Go directly to desired record by using a

key Computer does not have to read all prior

records Hashing algorithm used to determine

address of given key Requires disk storage

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Hashing Algorithm

Applies mathematical formula to key to determine disk address of given record Collision occurs when hashing algorithm

produces same disk address for two different keys

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Indexed File Organization

Combines elements of sequential and direct methods Records stored sequentially, but file also

contains an index Index stored sequentially, contains record

key Data accessed by record key

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Processing Stored Data

Transactions processed to update a master file Transactions - a business event such as a

sale Master file - data that is updated when a

transaction occurs, such as a sales file or inventory file

Two main methods of processing data Batch processing Transaction processing

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Batch Processing

Transactions collected into groups or batches Batch processed and

master file updated when the computer has few users online

Very efficient use of computer resources

Master file current only immediately after processing

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Transaction Processing

Processing transactions as they occur Also called real-time

processing and online processing

Terminals must be connected directly to the computer

Offers immediate updating of master file

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Objectives

List the benefits of secondary storage Identify and describe storage media that

are available for personal computers Differentiate among the principal types of

secondary storage Describe how data is stored on a disk Discuss the benefits of multimedia Explain how data is organized, accessed,

and processed