Module 6: Information storage

Objective– The bird view of data and information storage– Benefits of secondary storage– Second storage device for personal computers– Features of secondary storage devices. – How data is stored on a disk

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Domains and their measurement

• Space– Meter, millimeter, micrometer, nanometer

• Time– Second, millisecond, microsecond, nanosecond

• Frequency– Hertz (Hz), KHz, MHz, GHz– Clock cycle time = 1 / system clock frequency

• Information– Bit, Byte, KB, MB, GM, TB

• Printing/publication– Point (pt), 1 pt = 1/72 inch

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Storage• Hierarchy of data/information

storage– Register: small, fast – Cache: intermediate, not

addressable– Main memory: addressable,

volatile – Second storage, not

addressable, can only store files, e.g. program files, data files

• Benefit of secondary storage– Space– Reliability– Convenience– Economy

Control Unit

ALU

Registers

Main memory

CPU

Cache L1

Cache L2

Secondarystorage

Storage system

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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– A DVD disk can hold 3,500 books– The new 60 GB IPod can hold 15,000 songs

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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

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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Magnetic Disk Storage

• Data represented as magnetized spots on surface of spinning disk– Two states of magnetic molecule: Ordered or

unordered– Can be controlled, and changed by

electronic/magnetic field– 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

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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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Flash memory• Flash memory is a non-volatile memory

device that retains its data after the power is removed

• Made of electronic circuits, RAM-ROM hybrid, small with high capacity

• Access speed is between main memory and HD

• E.g. memory key, flesh memory in mobile device, IPod nano. Replacement to HD?

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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 on Disk

• 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: moving arm over a track– Head switching: from one head to another– Rotational delay: rotating to a sector

• Once data found, next step is data transfer

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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 thousandth of a second)

– 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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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

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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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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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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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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