1. Computers: Tools for an Information Age

2.  Separate from the computer itself  Software and data stored on a semipermanent basis  Unlike memory, not lost when power is lost  Benefits Benefits Space Reliability Convenience Economy Benefits of Secondary Storage 2

3. 3 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 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 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

4.  Data represented as magnetized spots on surface of spinning disk  Spots on disk converted to electrical impulses  Primary types  Diskettes- now replaced with USB drives Diskettes  Hard Disks Hard Disks  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 4

5.  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 5

6.  Each platter has its own access arm with read/write head  Most disk packs combine platters, access arms, and read/write head 6

7. 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 7

8. Redundant Array Of Independent Disks (RAID) 8  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

9. How Data Is Organized  Track Track  Sector Sector  Cluster Cluster  Cylinder Cylinder 9

10. 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 10

11. 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 11

12. 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 12

13. 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 13

14. Disk Access Speed  Access time - the time needed to access data on disk  Three factors  Seek time Seek time  Head switching Head switching  Rotational delay Rotational delay  Once data found, next step is data transferdata transfer 14

15. Seek Time  The time it takes the access arm to get into position over a particular track  All access arms move as a unit  All simultaneously in position over a set of tracks that make up a cylinder 15

16. Head Switching  The activation of a particular read/write head over a particular track  All access arms move together, but only one read/write head can operate at any one time 16

17. Rotational Delay  The time it takes for the desired data on the track to rotate underneath the read/write head  On average, half the time for a complete revolution of the disk 17

18. 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 thousands of a second)  Can be improved by disk cachingdisk caching  Data transfer rate - how fast data can be transferred once it has been found  Stated in terms of megabytes per second 18

19. Disk Caching  Disk cache - a special area of memory  When disk drive reads data from disk, it reads adjacent data and stores it in memory  When next read instruction is issued, drive checks first to see if desired data is in disk cache  Similar to memory caching discussed in Chapter 4 19

20. 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  Magneto-optical - combines magnetic and optical capabilities 20

21. 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 21

22. 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 22

23. Multimedia  Presents information with text, illustrations, photos, narration, music, animation, and film clips  Not practical until the advent of the optical disk  Requirements Requirements  Applications Applications 23

24. 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 24

25. Applications  Education  Go on virtual tours  Study musical scores  Study a foreign language  Other  Prepare taxes with video clips from IRS experts  Play games 25

26. 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 26

27. 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 27

28. Organizing and Accessing Stored Data  Character Character  Field Field  Record Record  File File  Database Database 28

29. Character  A letter, digit, or special character 29

30. Field  A set of related characters  Describes one characteristic of a person, place, or thing  For a university, a student’s first name would be stored in a field  Key field - a unique identifier for a record 30

31. Record  A collection of related fields  For the university, all of the fields for one student constitute one record 31

32. File  A collection of related records  For university, all the student records compose a file 32

33. Database  A collection of related files stored with minimum redundancy (duplication)  For university, student file, alumni file, faculty/staff file, courses file, financial file, etc. would make up a database  Organized to make retrieving data easier 33

34. 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 34

35. File Organization  Three major methods of organizing data files in secondary storage  Sequential Sequential  Direct Direct  Indexed Indexed 35

36. 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 36

37. 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 Hashing algorithm  Requires disk storage 37

38. Hashing Algorithm  Applies mathematical formula to key to determine disk address of given record  Collision occurs when hashing algorithm produces same disk address for two different keys 38

39. 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 39

40. 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 Batch processing  Transaction processing Transaction processing 40

41. 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 41

42. 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 42

43. Objectives  List the benefits of secondary storage  Identify and describe storage media that are available for personal computers  Differentiate among the principal types of secondary storage  Describe how data is stored on a disk  Discuss the benefits of multimedia  Explain how data is organized, accessed, and processed 43