Storage and Multimedia Part I

1. Storage and MultimediaPart I Bayram Güzer

2. 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

3. 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

4. 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

5. 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

6. 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.

7. 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.

8. 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.

9. 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

10. 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

11. 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

12. 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.

13. 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.

14. How Data Is Organized • Track • Sector • Cluster • Cylinder

15. 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

16. 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.

17. 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.

18. 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

19. 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

20. 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.

21. 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.

22. 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

23. 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.

24. 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

25. 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

26. 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

27. 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

28. 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.

29. 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

30. 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)

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