Storage and MultimediaPart I

Bayram Güzer

Secondary Storage

• Secondary storage is separate from the computer itself

• You can store software and data on a permanent basis– Unlike memory, data is not lost when power is

lost

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Benefits of Secondary Storage

• Space– You may 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

• Reliability– Data in secondary storage is relatively safe

• Secondary storage is physically reliable although sometimes fail.

• It is more difficult for untrained people to interfere with data stored on disk

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Benefits of Secondary Storage

• Convenience– Computers help authorized users to easily and quickly

locate data stored on the disk.

• 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 maintaining data

subject to errors• 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– Spots on disk is converted to electrical impulses

– Magnetized spot is representing 1 bit

– Absence of magnetized spot is representing 0 bit.

• Primary types– Diskettes

– Hard Disks

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Diskettes

• Made of flexible Mylar and coated with iron oxide which is a substance that can be magnetized.

• A diskette can record data as magnetized spots on tracks on its surface.

• Has protection of rigid plastic jacket and fits in your shirt pocket.

• Portability , easily transporting your data from one computer to another is one of the advantages of the diskettes.

• Diskettes are also convenient vehicle for backup.

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Diskettes

• 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

• High-capacity variation provides greater usability but if you intend to move a larger file, it can create problems to you.

• Data compression is the process of squeezing a big file into a small place.

• Compression is performed by a program that uses a formula to determine how to compress or decompress data.

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Hard Disks

• Hard disk is a metal platter coated with magnetic oxide that can be magnetized to represent data.– 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 is a separate unit

connected to the computer within a computer case.– Large computer systems may have dozens or even

hundreds of external disk drives– In a disk pack, all the discs rotate at the same time but

only one disk is being read or written from on at any one time.

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Reading/Writing Data

• Access arm moves read/write head over particular location

• Read/write head flies 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 9

Disk Packs

• A disk pack has a series of access arms that slips in between the disks in a pack.

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

• Two read/write heads are on each arm, one facing up to access the surface about it and one facing down to access the surface below it however only one head/write head can operate at any one time.

• Most disk packs are combined from platters, access arms, and read/write head

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Hard Disks for Personal Computers

• Hard disks for personal computers are 3 ½” disks in sealed modules.

• Capacity is measured in gigabytes– Nowadays terabyte capacity is on the

horizon.

• Accessing files is much faster than accessing files on diskettes

• Some contain removable cartridges– Iomega’s Jaz drive is very popular

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Redundant Array of Independent Disks (RAID)

• A group of small hard disks that work together as one.– Raid level 0 spreads data from a

single file over several drives• It is called as data striping

• One disk is used solely as a check disk to keep track of which data is where.

• Data is processed more quickly than other methods which increases performance of the computer.

• If a disk fails the check disk can reconstitute data.

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Redundant Array of Independent Disks (RAID)

• A group of small hard disks that work together as one.– Raid level 1 duplicates data on

several drives• It is called as disk mirroring

• Increases fault tolerance and no data is lost if one drive fails

• This method is reliable but expensive than others.

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How Data Is Organized

• 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– Data on the track is accessed by

surface number, track number, and sector number.

• There is a problem in this circular representation.– The distance around the tracks

on the outside of the disk is greater than that around the track on the inside.

– This means that the tracks on the outside are not storing data efficiently.

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Sector

• Zone recording takes maximum advantage of the storage available by dividing a disk into zones and assigning more sectors to tracks in outer zones than to those in inner zones.– Uses storage space more fully– Each sector on the disk holds

the same amount of data.– This means more data storage

than other method.17

Cluster

• Cluster is a fixed number of neighboring sectors that are treated as a unit of storage– Typically two to eight sectors, depending on

the operating system

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Cylinder

• Cylinder is another way of organizing data in a disk pack in comparison to sector method.

• The organization in this case is vertical.• The purpose is to reduce the time it takes

you to move the access arms of a disk pack into position.

• The track on each surface that is under 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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Cylinder

• Sector method (recording the data horizontally);– Start with first surface

• Fill track 000, 001, 002 and so on.

– Then move to second surface• Fill track 000, 001, 002 and so on.

– Each new track and new surface would require movement of the access arms.

• Cylinder method (recording the data vertically);– Data is recorded on the tracks that can be

accessed by one positioning of the access arms that is one cylinder.

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Cylinder

• Cylinder method means that – all tracks of a certain cylinder on a

disk pack are lined up one under the other

– and all the vertical tracks of one cylinder are accessible by the read/write heads

• with one positioning of the access arm mechanism.

– Tracks within a cylinder are numbered according to this vertical perspective, from 0 on the top down to the last surface on the bottom.

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Disk Access Speed

• Access time is the time needed to access data on disk

• There are three factors defining access time;– Seek time– Head switching– Rotational delay

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Three Factors

• Seek Time– This is the time it takes the access arm to get into

position over a particular track• All access arms move as a unit and• This makes all arms works simultaneously in position over a

set of tracks that make up a cylinder

• Head Switching– All access arms move together, but only one read/write

head can operate at any one time• The activation of a particular read/write head over a particular

track is called as head switching which takes time.

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Three Factors

• Rotational Delay– Once the access arm and read/write head are in

position and ready to read or write data, read/write head waits for a short time.

– The time it takes for the desired data on the track to rotate underneath the read/write head is called as rotational delay.

• Once data found, next step is data transfer

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Data Transfer• Data transfer is the process of transferring data

between its location on the disk track and memory– Reading process; from track to memory– Writing process; from memory to track

• Measures of performance– Average access time

• Usually measured in milliseconds• About 10 milliseconds (10 thousands of a second)• Can be improved by disk caching

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

• Stated in terms of megabytes per second

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Disk Caching

• Disk cache - a special area of memory– When disk drive reads data from disk, it reads

neighbouring data and stores it in memory– When next read instruction is issued, drive

checks first to see if desired data is in disk cache

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Optical Disk Storage

• Provides inexpensive and compact storage with greater capacity

• Writing data; – Heat from the laser produces tiny spots on the disk surface

• Reading data– Laser scans disk and picks up light reflections from the

various spots

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Optical Disk Storage• Categorized by read/write capability

– Read-only media: recorded by the manufacturer and the user can read from, but not write to disk

– Write-once, read-many (WORM) - user can write to disk once filled a worm disk becomes a read-only medium.

– Magneto-optical (hybrid type) - combines magnetic and optical capabilities.

• Has the high volume capacity of the optical disk but the data can be written over like a magnetic disk.

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Compact Disks

• Compact Disk – Read Only Memory stands for CD-ROM

• CD-ROM - drive can only read data from CDs– CD-ROM stores up to 700 MB per disk

(equivalent to 400 3 ½ diskettes)– 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 – Reading, writing, rewriting

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Digital Versatile Disk (DVD)

• Uses shorter wavelength laser to read densely packed (microscopic spots) and microscopic spots increases the capacity of the DVD– 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– Single sided single layer (4.76 GB)– Single sided dual layer (8.56 GB)– Double sided single layer (9.46 GB)– Double sided double layer (17 GB)

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References

• Capron, H. L. (2000). Computers Tools for an Information Age.(6th ed.). New Jersey, USA: Prentice Hall.

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