Storage and Multimedia: The Facts and More

Chapter 6

Objectives

• List the benefits of secondary storage

• Identify and describe storage media available for personal computers

• Differentiate among the principal types of secondary storage

• Discuss the benefits of multimedia

• Explain how data is organized, accessed, and processed

Contents

• The Benefits of Secondary Storage• Magnetic Disk Storage• Logical Layout of a Disk• Disk Drive Operation• Optical Disk• Magnetic Tape Storage• Organizing and Accessing Data• Processing Stored Data• Applications

Secondary Storage Benefits

• Semi-permanent

• Non-volatile

• Reliable

• Convenient – Locate and access data quickly

• Compressed storage– Diskette – about 500 printed pages – Optical disk – about 500 books

• Economy– Savings in physical storage costs– Savings in the speed and convenience of

filing and retrieving data

Secondary Storage Benefits

Types of Storage

• Magnetic Disk Storage

• Optical Disks– Magneto-optical– CD-ROM– CD-R– CD-RW– DVD-ROM

• Magnetic Tape Storage

Magnetic Disk Storage

• Data represented as magnetic spots– Magnetized spot = 1– Absence of a magnetized spot = 0

• Read– Converts the magnetized data to electrical

impulses

• Write– Converts electrical impulses to magnetized

spots on disk

Disk Capacity

Size

MBolder hard disks

GBcurrent PC

TBcoming soon

What’s stored?

User documents

Software

Graphic images

Audio files

Video files

Diskettes

• Low capacity – small files

• Portable

• Flexible Mylar coated with metallic substance

• Hard plastic jacket for protection

• 3 ½ inch, 1.44 MB

High-Capacity Portable Disks

• Larger files

• Portable

• High-capacity– 120 / 200 MB– Can read and write standard diskettes– Ex: Superdisk

• Zip disk– 250 MB– not compatible with 3 ½ inch diskettes

Data Compression• Why use?

– Squeeze big files onto small disks– Speed up data transfer of files

• Techniques– Remove all extra space characters– Substitutes a smaller data string for a

frequently occurring set of characters– Software uses formula to determine how to

compress– Must be decompressed

Hard Disk

• Various sizes

• Portability– Generally non-portable– Removable hard disks available for PC

• Rigid platter coated with metallic substance

Disk Pack

Several platters

Airtight, sealed module

Mount disk pack on disk drive

Disk Pack

• Disk pack has set of access arms

• Two read / write heads per arm– One reads top surface– One reads bottom surface

• Access arms move together as a unit

• Only one read/write head works at a time

Logical Layout of a DiskTrack

• Concentric circles

• Passes under read/write head as disk rotates

• 1.44 MB diskette has 80 tracks on each surface

• Each track stores the same amount of data

Logical Layout of a DiskSector

• Pie-shaped division of track

• Holds a fixed number of bytes (512 bytes)

• Cluster– Adjacent sectors treated as a unit of storage– Fixed number (2-8 sectors)– Minimum space allocated to a file

• Same track on each platter

• Store files across multiple platters

• Reduces access time

Logical Layout of a DiskCylinder

Logical Layout of a DiskZone Recording

• Assigns more sectors to tracks in outer zones

• More sectors = more data storage available

Disk DriveRead / Write Operation

• Disks rotate

• Access arm moves read/write head

• Read / write operation begins and continues until complete

• Data is transferred to/from memory

Access Time

Seek time

Head switching

Rotational delay

Data transfer rate

Data DestroyedHead Crash

Disk Caching

• Required data read into memory

• Adjacent data read into disk cache (special area of memory)

• Program encounters a read instruction– Check disk cache– If present, no physical read is required– If not present, read from disk

RAID

Redundant Array of Independent Disks

Optical Disk

• Greater capacity than other portable media

• Process– Laser writes on metallic material spread over the

surface of disk– Heat from laser produces pits on disk surface– Reading – laser picks up light reflections from the

pits

• Technology– ROM– WORM

MOMagneto-optical

• Hybrid

• High-volume capacity

• Written multiple times

• Process– Laser melts a microscopic spot– Magnet aligns crystals– Reading – laser picks up light reflection from

crystals

CD-ROMCompact Disk Read-Only Memory

• High capacity portable

• Read multiple times

• Cannot record

• Capacity – up to 680 MB (450 standard 3 ½ inch diskettes)

• Used for software distribution

CD-RCompact Disc-Recordable

• High capacity

• Portable

• Write once

• Read multiple times– CD-R drive– CD-ROM drive

CR-RWCompact Disk-Rewritable

• High capacity

• Portable

• Read multiple times

• Record multiple times

• Some compatibility problems reading CD-RW disks on CD-ROM drives

DVD-ROMDigital Versatile Disk

• Larger capacity than CD-ROM– Standard – Up to 4.7 GB, 7 times more than

CD-ROM– Double layers – 8.5 GB– Double-sided – 17 GB

• Data is packed more densely

• Read multiple times, Cannot record

• Can read CD-ROM disks

• Benefits– Full-length movies– Audio quality comparable to audio compact

disks– High-volume business data

• Expected to replace CD-ROM in the near future

DVD-ROMDigital Versatile Disk

Magnetic Tape Storage

• Plastic tape with magnetic coating

• Capacity based on density – bpi or cpi

• Magnetic tape unit– Read/write head– Erase head erases previously recorded data

• Inferior to disks– Not as reliable– Sequential access to data

• Inexpensive

• Primarily for backup

Backup Systems

• Prevent data loss– Fire– Natural disaster– Electromechanical failures of disk– User introduced errors– Software errors– Accidental data deletion

• Store data in more than one place

DataOrganizing and Accessing

• Plan for way data is– Received– Organized– Stored– how it will be processed

• Plan determined by programmer or systems analyst

DataGetting Organized

Character

Field

Record

File

Database

DataGetting Organized

Key Field

Unique identifier for a record

Data Access Methods

• Application determines how data must be accessed by users

• Data is organized based upon access method

• Organization method limits choice of storage medium

Sequential

• Records are stored and accessed in order

• All records prior to the one requested must be read

• Magnetic tape storage

Direct / Random Access

• Records are not physically stored in any order

• Go directly to the record to read– Hashing – apply a formula to the key to produce

the address of the record– Collision – same address from different keys

• Updating in place – Read, change, and return a record to the same

place on disk

• DASD – Direct-Access Storage Device needed

Indexed• Records are stored sequentially• Index is generated that contains key and address• Can be read in order = sequential• Can be read out of order = random

Processing Stored Data

• Batch

• Transaction

• Terminology– Transaction – updates a record– Master file – contains all the data

Processing Stored Data

Batch

• Collect transactions into a transaction file and perform periodic updates

• Process– Transactions are sorted by key field– Computer matches the master and transaction

keys– Performs requested action – add, revise, delete– New master file created– Error report is printed

• Master file only current immediately after processing

Transaction

• Processed upon request

• Real-time – process handled immediately

• Disk storage– Direct access to desired record needed– Immediate access to stored data– Immediate updating of stored data

Batch and Transaction

• Computer system may use both processing types based upon the application

• Transaction– Activities relating to current needs

• Batch– Updates per schedule

ApplicationsBank

• Transaction– Check balance– Record cash withdrawal

• Batch– Deposit left in the deposit drop– Bank statement

ApplicationsRetail – POS

• Transaction– Item price– Inventory updates as sale is made

• Batch– Produce daily and weekly sales reports

ApplicationsMotor Vehicle

• Transaction– Police check for stolen car report

• Batch– Motor vehicle records of owner information

• Hardware– CD-ROM or DVD-ROM– Sound card or sound chip– Speakers

• MPEG– Video standards that support full-motion

video– Faster drive provides faster data transfer

and produces a smoother video

ApplicationsMultimedia