Secondary Storage

Secondary Storage

l   Separate from the computer itself

l   Software and data stored on a semi-permanent basis

l   Unlike memory, not lost when power is lost

Storage Media & Storage Devices

l   Physical components or materials on which data is stored are called storage media.

l   The hardware components that write data to and read it from storage media are called storage devices.

l   Diskette – Storage Medium

l   Diskette Drive – Storage device

Types of storage technology


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

l   Diskette contains equivalent of 500 printed pages

l   Optical disk can hold equivalent of 500 books


l   Data in secondary storage is relatively safe

l   Secondary storage is highly reliable

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


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

Magnetic disk storage

l   Technology of representing data as magnetized spots on the disk

l   With a magnetized spot represents 1 bit and absence of spot represents 0 bit.

l   Reading data from the disk means converting the magnetized data to electronic impulses that can be sent to the processor

l   Writing data is sending impulses from the processor to be converted to magnetized spots on the disk.

Magnetic disk storage

l   Surfaces of disks and magnetic tapes are coated with millions of tiny iron particles so that data can be stored on them.

l   Each of these particles can act as magnet, taking on a magnetic field when subjected to an electromagnet.

l   The read/write head of the hard drive contains electromagnet which generates magnetic field in the iron on the storage medium as the head passes over the tape or disk.

Disk Capacity



older hard disks


current PC


coming soon

What’s stored?

User documents


Graphic images

Audio files

Video files


l   A diskette is made of flexible Mylar and coated with iron oxide which can be magnetized.

l   During the 1980 most PC used 5.25” diskettes. 3.5” refers to diameter of the disks.


l    Low capacity – small files

l    Portable

l    Flexible Mylar coated with metallic substance

l    Hard plastic jacket for protection

l    3 ½ inch, 1.44 MB


l    Made of flexible Mylar and coated with iron oxide

l    Has protection of rigid plastic jacket

l    3 ½” diskette holds 1.44 MB of data

l    High-capacity variations

l   Sony’s HiFD holds 200 MB

l   Imation’s SuperDisk available in 120 and 240 MB versions

l   Iomega’s Zip drive available in 100, 250, and 750 MB versions

Hard Disks

l   Rigid platter coated with magnetic oxide

l   Several can be combined into a disk pack

l   Disk drive – a device that allows data to be read from or written to a disk

l   Disk drive for personal computers contained within computer housing

l   Large computer systems may have several external disk drives

Reading/Writing Data

l    Access arm moves read/write head over particular location

l    Read/write head hovers a few millionths of an inch above platter

l   If head touches platter, a head crash occurs and data is destroyed

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

Disk Packs

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

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

Hard Disks for Personal Computers

l    Sealed modules that mount in a 3 ½” bay

l    Capacity in gigabytes

l    Accessing files much faster than accessing files on diskettes

l    Some contain removable cartridges

l   Iomega’s Jaz drive is very popular

Redundant Array of Independent Disks (RAID)

l    A group of disks that work together as one

l   Raid level 0 spreads data from a single file over several drives

l   Called data striping

l   Increases performance

l   Raid level 1 duplicates data on several drives

l   Called disk mirroring

l   Increases fault tolerance

RAID (Redundant array of independent disks)

l   Uses several small hard disks that work together as a unit.

l   Raid level 1 (Disk mirroring) – simply copies data on separate disk drives. Reliable but expensive

Data Striping with RAID

l   Higher level of RAID

l   Data striping involves spreading the data across several disks with one disk acts as a check disk to keep track of data.

l   Quickly process data.

Organization of data in disks

l   The Sector Method

Used for diskettes

l   The cylinder method

Used for hard disks

How Data Is Organized

l   Track

l   Sector

l   Cluster

l   Cylinder


l    The circular portion of the disk surface that passes under the read/write head

l   Floppy diskette has 80 tracks on each surface

l   Hard disk may have 1,000 or more tracks on each surface of each platter


l    Each track is divided into sectors that hold a fixed number of bytes

l   Typically 512 bytes per sector

l    Zone recording assigns more sectors to tracks in outer zones than those in inner zones

l   Uses storage space more fully


l   A fixed number of adjacent sectors that are treated as a unit of storage

l   Typically two to eight sectors, depending on the operating system


l    The track on each surface that is beneath the read/write head at a given position of the read/write heads

l   When file is larger than the capacity of a single track, operating system will store it in tracks within the same cylinder

Disk Access Speed

l   Access time – the time needed to access data on disk

l   Three factors

l   Seek time

l   Head switching

l   Rotational delay

l   Once data found, next step is data transfer

Seek Time

l   The time it takes the access arm to get into position over a particular track

l   All access arms move as a unit

l   All simultaneously in position over a set of tracks that make up a cylinder

Head Switching

l   The activation of a particular read/write head over a particular track

l   All access arms move together, but only one read/write head can operate at any one time

Rotational Delay

l   The time it takes for the desired data on the track to rotate underneath the read/write head

l   On average, half the time for a complete revolution of the disk

Data Transfer

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

l   Measures of performance

l   Average access time

l  About 10 milliseconds (10 thousands of a second)

l  Can be improved by disk caching

l   Data transfer rate – how fast data can be transferred once it has been found

l  Stated in terms of megabytes per second

Difference between hard disks and diskettes

l     Diskette contains a single, flat piece of plastic, coated with iron oxide and enclosed in a vinyl or plastic cover. A hard disk contains one or more rigid platters, coated with iron oxide, which are permanently encased in the hard disk dive.

l     Diskettes are small and portable but in general, hard disks are not. Exceptions are removable hard drive and external hard drive.

l     Most floppy disks store 1.44 MB of data while hard disks can store several thousand times as much data as a diskette.

l     Hard disk drives are much faster than diskette drives.

Tape Drive

l   Read and write data to the surface of a tape the same way an audio cassette recorder does.

l   Computer tape drive writes digital data.

l   Usually used to store data that is not used frequently- back up of your hard drive

l   Is an inexpensive way to store large amount of data.

Logical Layout of a Disk

l   Same track on each platter

l   Store files across multiple platters

l   Reduces access time

Logical Layout of a Disk
Zone Recording

Disk Drive
Read / Write Operation

l     Disks rotate

l     Access arm moves read/write head

l     Read / write operation begins and continues until complete

l     Data is transferred to/from memory

Organizing and Accessing Stored Data

l    Character

l    Field

l    Record

l    File

l    Database


l   A letter, digit, or special character


l   A set of related characters

l   Describes one characteristic of a person, place, or thing

l   For a university,  a student’s first name would be stored in a field

l   Key field – a unique identifier for a record


l   A collection of related fields

l   For the university, all of the fields for one student constitute one record


l   A collection of related records

l   For university, all the student records compose a file


l   A collection of related files stored with minimum redundancy (duplication)

l   For university, student file, alumni file, faculty/staff file, courses file, financial file, etc. would make up a database

l   Organized to make retrieving data easier

Optical Disk

l   Greater capacity than other portable media

l   Process

l   Laser writes on metallic material spread over the surface of disk

l   Heat from laser produces pits on disk surface

l   Reading – laser picks up light reflections from the pits

l   Technology

l   ROM

l   WORM


l   Hybrid

l   High-volume capacity

l   Written multiple times

l   Process

l   Laser melts a microscopic spot

l   Magnet aligns crystals

l   Reading – laser picks up light reflection from crystals

Magnetic disk storage

l   Surfaces of disks and magnetic tapes are coated with millions of tiny iron particles so that data can be stored on them.

l   Each of these particles can act as magnet, taking on a magnetic field when subjected to an electromagnet.

l   The read/write head of the hard drive contains electromagnet which generates magnetic field in the iron on the storage medium as the head passes over the tape or disk.

Optical Storage Devices –
How Optical Storage Works

•      An optical disk is a high-capacity storage medium.  An optical drive uses reflected light to read data.

•      To store data, the disk’s metal surface is covered with tiny dents (pits) and flat spots (lands), which cause light to be reflected differently.

•      When an optical drive shines light into a pit, the light cannot be reflected back.  This represents a bit value of 0 (off).  A land reflects light back to its source, representing a bit value of 1 (on).

Compact Disk Read-Only Memory

l    High capacity portable

l    Read multiple times

l    Cannot record

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

l    Used for software distribution

CD ROM Speed

l     Compare to hard disk drive, CD Rom drives are quite slow in part because the laser reads bits and lands one bit at a time.

l     CD Rom Drive has to adjust the speed while reading data in the middle and in the edge.

l     Changing the speed of rotation takes time.

l     The first CD Rom Drive reads data at 150 KBps.

l     Now CD Rom drive reads data two (300 KBps) to fifty two(7800 KBps) times faster than the first model

CD ROM Users

l     Data that are rarely changed. (dictionaries, encyclopedias.)

l     Software companies can distribute their product on CD ROM.

Compact Disc-Recordable

l    High capacity

l    Portable

l    Write once

l    Read multiple times

l   CD-R drive

l   CD-ROM drive

Compact Disk-Rewritable

l   High capacity

l   Portable

l   Read multiple times

l   Record multiple times

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

Digital Versatile Disk

l   Larger capacity than CD-ROM

l   Standard – Up to 4.7 GB, 7 times more than CD-ROM

l   Double layers – 8.5 GB

l   Double-sided – 17 GB

l   Data is packed more densely

l   Read multiple times, Cannot record

l   Can read CD-ROM disks

Digital Versatile Disk

l   Benefits

l   Full-length movies

l   Audio quality comparable to audio compact disks

l   High-volume business data

l   Expected to replace CD-ROM in the near future

Magnetic Tape Storage

l   Plastic tape with magnetic coating

l   Capacity based on density – bpi or cpi

l   Magnetic tape unit

l   Read/write head

l   Erase head erases previously recorded data

l   Inferior to disks

l   Not as reliable

l   Sequential access to data

l   Inexpensive

l   Primarily for backup

Backup Systems

l   Prevent data loss

l   Fire

l   Natural disaster

l   Electromechanical failures of disk

l   User introduced errors

l   Software errors

l   Accidental data deletion

l   Store data in more than one place

Organizing and Accessing

l   Plan for way data is

l   Received

l   Organized

l   Stored

l   how it will be processed

l   Plan determined by programmer or systems analyst

Getting Organized






Getting Organized

Key Field

Unique identifier for a record

Data Access Methods

l   Application determines how data must be accessed by users

l   Data is organized based upon access method

l   Organization method limits choice of storage medium


l   Records are stored and accessed in order

l   All records prior to the one requested must be read

l   Magnetic tape storage

Direct / Random Access

l     Records are not physically stored in any order

l     Go directly to the record to read

l    Hashing – apply a formula to the key to produce the address of the record

l    Collision – same address from different keys

l     Updating in place

l    Read, change, and return a record to the same place on disk

l     DASD – Direct-Access Storage Device needed


l     Records are stored sequentially

l     Index is generated that contains key and address

l     Can be read in order = sequential

l     Can be read out of order = random

Processing Stored Data

l   Batch

l   Transaction

l   Terminology

l   Transaction – updates a record

l   Master file – contains all the data

Processing Stored Data


l   Collect transactions into a transaction file and perform periodic updates

l   Process

l   Transactions are sorted by key field

l   Computer matches the master and transaction keys

l   Performs requested action – add, revise, delete

l   New master file created

l   Error report is printed

l   Master file only current immediately after processing


l     Processed upon request

l     Real-time – process handled immediately

l     Disk storage

l    Direct access to desired record needed

l    Immediate access to stored data

l    Immediate updating of stored data

Batch and Transaction

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

l   Transaction

l   Activities relating to current needs

l   Batch

l   Updates per schedule


l   Transaction

l   Check balance

l   Record cash withdrawal

l   Batch

l   Deposit left in the deposit drop

l   Bank statement

Retail – POS

l   Transaction

l   Item price

l   Inventory updates as sale is made

l   Batch

l   Produce daily and weekly sales reports

Motor Vehicle

l   Transaction

l   Police check for stolen car report

l   Batch

l   Motor vehicle records of owner information


l    Hardware


l   Sound card or sound chip

l   Speakers

l    MPEG

l   Video standards that support full-motion video

l   Faster drive provides faster data transfer and produces a smoother video