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
