Transcript Computers: Tools for an Information Age

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
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
Storage system
Secondary
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
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Main memory
Cache L2
Cache L1
Control Unit
Space
Reliability
Convenience
Economy
Registers
ALU
CPU
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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
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•
•
•
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
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