1Module 6: Storage How do you explain how CPU works to a housewife

2Module 6: Storage Domains and their measurement Space Meter, millimeter, micrometer, nanometer Meter, millimeter, micrometer, nanometerTime Second, millisecond, microsecond, nanosecond Second, millisecond, microsecond, nanosecondFrequency Hertz (Hz), KHz, MHz, GHz Hertz (Hz), KHz, MHz, GHz Clock cycle time = 1 / system clock frequency Clock cycle time = 1 / system clock frequencyInformation Bit, Byte, KB, MB, GM, TB Bit, Byte, KB, MB, GM, TBPrinting/publication Point (pt), 1 pt = 1/72 inch Point (pt), 1 pt = 1/72 inch

Copyright © 2003 by Prentice Hall Module 6: Information storage Objective The bird view of data and information storage The bird view of data and information storage Benefits of secondary storage Benefits of secondary storage Second storage device for personal computers Second storage device for personal computers Features of secondary storage devices. Features of secondary storage devices. How data is stored on a disk How data is stored on a disk

4Module 6: Storage Storage Hierarchy of data/information storage Register: small, fast Register: small, fast Cache: intermediate, not addressable Cache: intermediate, not addressable Main memory: addressable, volatile Main memory: addressable, volatile Second storage, not addressable, can only store files, e.g. program files, data files Second storage, not addressable, can only store files, e.g. program files, data files Control Unit ALU Registers Main memory CPU Cache L1 Cache L2 Secondary storage Storage system

5Module 6: Storage Benefits of Secondary Storage SpaceReliabilityConvenienceEconomy

6Module 6: Storage Space Store a roomful of data on disks smaller than the size of a breadbox Diskette contains equivalent of 500 printed pages Diskette contains equivalent of 500 printed pages Optical disk can hold equivalent of 500 books Optical disk can hold equivalent of 500 books A DVD disk can hold 3,500 books A DVD disk can hold 3,500 books

7Module 6: Storage Reliability Data in secondary storage is relatively safe Secondary storage is highly reliable Secondary storage is highly reliable More difficult for untrained people to tamper with data stored on disk More difficult for untrained people to tamper with data stored on disk

8Module 6: Storage Convenience Authorized users can easily and quickly locate data stored on the computer

9Module 6: Storage Economy Several factors create significant savings in storage costs Less expensive to store data on disks than to buy and house filing cabinets Less expensive to store data on disks than to buy and house filing cabinets Reliable and safe data is less expensive to maintain Reliable and safe data is less expensive to maintain Greater speed and convenience in filing and retrieving data Greater speed and convenience in filing and retrieving data

10Module 6: Storage Magnetic Disk Storage Data represented as magnetized spots on surface of spinning disk Two states of magnetic molecule: Ordered or unordered Two states of magnetic molecule: Ordered or unordered Can be controlled, and changed by electronic/magnetic field Can be controlled, and changed by electronic/magnetic field Spots on disk converted to electrical impulses Spots on disk converted to electrical impulses Primary types Diskettes Diskettes Hard Disks Hard Disks

11Module 6: Storage 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 Sony’s HiFD holds 200 MB Imation’s SuperDisk available in 120 and 240 MB versions Imation’s SuperDisk available in 120 and 240 MB versions Iomega’s Zip drive available in 100, 250, and 750 MB versions Iomega’s Zip drive available in 100, 250, and 750 MB versions

12Module 6: Storage Hard Disks Rigid platter coated with magnetic oxide Several can be combined into a disk pack 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 Disk drive for personal computers contained within computer housing Large computer systems may have several external disk drives Large computer systems may have several external disk drives

13Module 6: Storage 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 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 Data can be destroyed if head touches miniscule foreign matter on surface of disk

14Module 6: Storage Disk Packs Each platter has its own access arm with read/write head Most disk packs combine platters, access arms, and read/write head

15Module 6: Storage 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 Iomega’s Jaz drive is very popular

16Module 6: Storage 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 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 Raid level 1 duplicates data on several drives Called disk mirroring Increases fault tolerance

17Module 6: Storage How Data Is Organized on Disk TrackSectorClusterCylinder

18Module 6: Storage Track The circular portion of the disk surface that passes under the read/write head Floppy diskette has 80 tracks on each surface Floppy diskette has 80 tracks on each surface Hard disk may have 1,000 or more tracks on each surface of each platter Hard disk may have 1,000 or more tracks on each surface of each platter

19Module 6: Storage Sector Each track is divided into sectors that hold a fixed number of bytes Typically 512 bytes per sector 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 Uses storage space more fully

20Module 6: Storage 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 Typically two to eight sectors, depending on the operating system

21Module 6: Storage 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 When file is larger than the capacity of a single track, operating system will store it in tracks within the same cylinder

22Module 6: Storage Disk Access Speed Access time - the time needed to access data on disk Three factors Seek time: moving arm over a track Seek time: moving arm over a track Head switching: from one head to another Head switching: from one head to another Rotational delay: rotating to a sector Rotational delay: rotating to a sector Once data found, next step is data transfer

23Module 6: Storage Data Transfer The process of transferring data between its location on the disk track and memory Measures of performance Average access time 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 Data transfer rate - how fast data can be transferred once it has been found Stated in terms of megabytes per second

24Module 6: Storage 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 Read-only media - user can read from, but not write to disk Write-once, read-many (WORM) - user can write to disk once Write-once, read-many (WORM) - user can write to disk once

25Module 6: Storage Compact Disks CD-ROM - drive can only read data from CDs CD-ROM stores up to 700 MB per disk CD-ROM stores up to 700 MB per disk Primary medium for software distribution Primary medium for software distribution CD-R - drive can write to disk once Disk can be read by CD-ROM or CD-R drive 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 Some compatibility problems trying to read CD-RW disks on CD-ROM drives

26Module 6: Storage Digital Versatile Disk (DVD) Short wavelength laser can read densely packed spots DVD drive can read CD-ROMs DVD drive can read CD-ROMs Capacity up to 17GB Capacity up to 17GB Allows for full-length movies Allows for full-length movies Sound is better than on audio CDs Sound is better than on audio CDs Several versions of writable and rewritable DVDs exist

27Module 6: Storage Multimedia Presents information with text, illustrations, photos, narration, music, animation, and film clips Not practical until the advent of the optical disk RequirementsApplications

28Module 6: Storage Requirements CD-ROM or DVD-ROM drive Sound card or sound chip Speakers For high-quality sound, get good speakers and powered subwoofer For high-quality sound, get good speakers and powered subwoofer Equipped to handle MPEG Standards for compressing video Standards for compressing video

29Module 6: Storage Magnetic Tape Storage Tape similar to tape used in music cassettes Categorized in terms of density Number of bits per inch stored on tape Number of bits per inch stored on tape Used primarily for backup of data stored on disk systems

30Module 6: Storage Backup Systems Imperative to have copies of important data stored away from the computer Disks occasionally fail Disks occasionally fail Software installation can cause computer to crash Software installation can cause computer to crash Users make mistakes entering data Users make mistakes entering data Tape is ideal backup medium Can copy entire hard disk to single tape in minutes Can copy entire hard disk to single tape in minutes Backup can be scheduled when you are not going to use the system Backup can be scheduled when you are not going to use the system

31Module 6: Storage File Plan Overview Must devise a plan for placing data on a storage unit Key factors Whether users must access data directly (immediately) Whether users must access data directly (immediately) How data must be organized on disk How data must be organized on disk Type of processing that will take place Type of processing that will take place

32Module 6: Storage File Organization Three major methods of organizing data files in secondary storage Sequential Sequential Direct Direct Indexed Indexed

33Module 6: Storage 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 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 To update a record, a new sequential file must be created, with changed and unchanged records Tape storage uses sequential organization

34Module 6: Storage 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 Computer does not have to read all prior records Hashing algorithm used to determine address of given key Hashing algorithm used to determine address of given key Requires disk storage

35Module 6: Storage Indexed File Organization Combines elements of sequential and direct methods Records stored sequentially, but file also contains an index Records stored sequentially, but file also contains an index Index stored sequentially, contains record key Index stored sequentially, contains record key Data accessed by record key Data accessed by record key

36Module 6: Storage Processing Stored Data Transactions processed to update a master file Transactions - a business event such as a sale 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 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

37Module 6: Storage Batch Processing Transactions collected into groups or batches Batch processed and master file updated when the computer has few users online 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

38Module 6: Storage Transaction Processing Processing transactions as they occur Also called real-time processing and online processing Also called real-time processing and online processing Terminals must be connected directly to the computer Terminals must be connected directly to the computer Offers immediate updating of master file