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Chapter 6

Chapter 6. Storage and Multimedia: The Facts and More. Benefits of Secondary Storage. Space Reliability Convenience Economy. Magnetic Disk Storage. Data represented in magnetic spots Each spot is one bit Magnetized spot = 1 Non-magnetized spot = 0

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Chapter 6

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  1. Chapter 6 Storage and Multimedia: The Facts and More

  2. Benefits of Secondary Storage • Space • Reliability • Convenience • Economy

  3. Magnetic Disk Storage • Data represented in magnetic spots • Each spot is one bit • Magnetized spot = 1 • Non-magnetized spot = 0 • Hard disks, zip disks, and floppy disks are magnetic storage

  4. Optical Disk Storage • Data written and read with a laser • Write-once, read-many (WORM) • commonly used for backup • Some forms of optical storage allow the user to write multiple times

  5. Compact Disks (Optical) • Compact Disk Read-Only Memory (CD-ROM) • 700 MB – about 450 floppies worth of storage • Compact Disk-Recordable (CD-R) • Write (record) once • Compact Disk-Rewritable (CD-RW) • Record multiple times • Many PC’s now come with CD-R or CD-RW drives

  6. Digital Versatile Disk (Optical) • Digital Versatile Disk Read-Only Memory (DVD-ROM) • 4.7 GB – about the same as 7 CDs • common storage format for movies • replaces VHS tape format • Writable version of Digital Versatile Disk (DVD-RAM)

  7. Magnetic Tape Storage • Data represented in magnetic spots • Primarily for backup • Density • cpi = characters per inch • bpi = bits per inch • Inexpensive • Sequential access to data • Slow

  8. Diskette • Flexible plastic with metallic coating • 3 ½ inch • 1.44 MB • Low capacity – small files • Hard plastic jacket • Portable

  9. High-Capacity Portable Disks • Can hold larger files • Zip disk - 100 MB or 250 MB • Super Disk - 120 MB • Jazz Disk - 1 or 2 GB • Data compression can be used to store more data in the same area

  10. Hard Disk • Rigid platter with metallic coating • Typical PC has 20 GB to 100 GB hard drive • Non-portable, permanently mounted inside the system unit

  11. Redundant Array of Independent Disks (RAID) • Disk mirroring • Simplest form of RAID • reduces possibility of data loss • maintains multiple copies of the same data on different drives • Data striping • Higher level of RAID • Spreads data across several disks • Uses check disk to rebuild lost data

  12. How Disk Drives Work • Disks rotate, then access arms move read/write heads in or out • All access arms move together • Only one read/write head can operate at a time • Read = Data transfer from the drive to memory • Write = Data transfer from memory to the drive

  13. Logical Layout of a Disk • Tracks • Concentric circles • Standard floppy has 80 tracks on each side • Hard disks may have 1000 or more tracks per surface

  14. Logical Layout of a Disk • Sector • Fixed size: usually 512 bytes Zone recording assigns more sectors to outer tracks ↓↓↓↓↓↓

  15. Logical Layout of a Disk • Cluster • 2 to 8 adjacent sectors • Data is read into memory one cluster at a time • Data is stored one cluster at a time • # of sectors in a cluster depends on the OS

  16. Logical Layout of a Disk • Cylinder • Same track on each platter • Large files stored in cylinder • Access arms all move together • Faster access if data is on the same cylinder

  17. Disk Access Speed • Access time (3 factors) • Seek time • Head switching • Rotational delay • Data transfer rate • Disk cache

  18. Data Organization • Character – letter, digit, or symbol ($, ?, *, etc) • Field – set of related characters • Record – collection of related fields • Key field – a field that is different for each record • File – collection of records • Database – collection of files

  19. Data Access • Sequential • Records stored in order by a key field • Slow retrieval • Used on tape storage

  20. Data Access • Direct • Also called random access • Hashing algorithm determines the address of a record • Collision – if an address is already used, another record must find a different address • Cannot be used on tape

  21. Data Access • Indexed • Compromise between sequential and direct • Records stored sequentially • An index tells where to find each key

  22. Data Processing Batch processing • Collects transactions and processes them later • Master file • Transaction file • Used when changes don’t have to be processed immediately • Bank statements, Payroll, etc

  23. Data Processing Transaction processing • Real-time, happens now • Online – computers / terminals connected directly

  24. Multimedia Hardware Requirements • CD-ROM, DVD-ROM • Drive speed • Sound card • Speakers • MPEG – Motion Picture Experts Group • Widely accepted format for storing video

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