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Computers: Tools for an Information Age

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Computers: Tools for an Information Age
Chapter 6 Storage and Multimedia: The Facts and More

Copyright © 2003 by Prentice Hall

Secondary Storage
• Separate from the computer itself • Software and data stored on a semipermanent basis
– Unlike memory, not lost when power is lost

• Benefits

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Benefits of Secondary Storage
• • • • Space Reliability Convenience Economy

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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

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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

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

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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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Copyright © 2003 by Prentice Hall 7

Magnetic Disk Storage
• Data represented as magnetized spots on surface of spinning disk
– Spots on disk converted to electrical impulses

• Primary types
– Diskettes – Hard Disks

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Diskettes
• Made of flexible Mylar and coated with iron oxide • Has protection of rigid plastic jacket • 3 ½” diskette holds 1.44 MB of data • High-capacity variations
– Sony’s HiFD holds 200 MB – Imation’s SuperDisk available in 120 and 240 MB versions – Iomega’s Zip drive available in 100, 250, and 750 MB versions Return
Copyright © 2003 by Prentice Hall 9

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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Copyright © 2003 by Prentice Hall 16

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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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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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

Copyright © 2003 by Prentice Hall

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Organizing and Accessing Stored Data
• • • • • Character Field Record File Database

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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

Copyright © 2003 by Prentice Hall

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File Organization
• Three major methods of organizing data files in secondary storage
– Sequential – Direct – Indexed

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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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Copyright © 2003 by Prentice Hall 44

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

Copyright © 2003 by Prentice Hall

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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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Copyright © 2003 by Prentice Hall 47

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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Copyright © 2003 by Prentice Hall 48

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