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A Secondary Storage: CD – ROM

A Secondary Storage: CD – ROM. Dr. Robert J. Hammell Assistant Professor Towson University Computer and Information Sciences Department 8000 York Road - Suite 406 Towson, MD  21252 http://triton.towson.edu/~rhammell/. CD-ROM. Compact Disc Read-Only Memory No longer just read only !!

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A Secondary Storage: CD – ROM

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  1. A Secondary Storage:CD – ROM Dr. Robert J. Hammell Assistant Professor Towson University Computer and Information Sciences Department 8000 York Road - Suite 406 Towson, MD  21252 http://triton.towson.edu/~rhammell/

  2. CD-ROM • Compact Disc • Read-Only Memory • Nolonger just read only!! • Recordable CDs are now common • Cheaper • Faster • DVD is also available for optical disks • Digital Video Disc • Digital Versatile Disk

  3. CD challenges • Strengths • Highstorage capacity • Inexpensive price • Durability • Weakness • Seek performance is slow • In library analogy • Index lookup  20 seconds • Equivalent disk access  58 days • CD analogy  more than 2 ½ years!!

  4. Physical Organization of CDs • Started as medium for audio • Designed for music • Highcapacity • Moderatetransferrate • Not designed forfast, random data access • No concerns for fast seek performance • Good random access performance must come from filestructuredesign!

  5. Pits and lands • Data written with a laser • Pits: coating changed by the laser • Lands: areas between the pits • Transition from a pit to land and back

  6. CLV versus CAV • Constant linear velocity • Uses single, spiral track • Packs data tightly • Spins more slowly at outer edges than at inner • Constant angular velocity • Normal concentric tracks and pie-shaped sectors • Write data less densely in the outer tracks • Tradeoff:largestorage versus seektime • CAV would have almost half present capacity • CLV seeking uses trial-and-error speed control

  7. Speed notes • Speeds given in relation to original audio CD speeds • “single-speed” – 150KBps • Other speeds are multiples of this • 48-speed or 48x means 48timesfaster • Like Hertz commercial – “notexactly”! • About 19x on innermost tracks • Only 48x on the outertracks • Exception: • TrueXtechnology from ZenResearch (Kenwood) • 52x gets 45x-52x across the entiredisk

  8. CD-RW • CD-ReWritable • User can record files, delete files, etc. • Recording layer is different • Crystallinecompound of silver, indium, antimony, tellurium • Heat to onetemp& cool  reflective • Heat to highertemp & cool  absorptive • Laser has 3 powers to use on recording layer • High power (write) creates absorptive places • Medium power (erase) converts to reflective state • Low power (read) does not alter the recording layer • Speed: 40x/12x/48x CD-R write CD-RW write Note: CD-R means CD-Recordable Read

  9. DVD • Digital Video (or Versatile) Disc • Samesize as a CD • 7-foldincrease in capacity • Largely due to improvements in technology • (1) Closer tracks; smaller pits CD DVD

  10. (2) More than onelayer of data is possible • (3) Can use double-sided disks • (4) More efficient structure of data • Error correction code is better • Needs lessbits moreroom for data

  11. CD-ROM Strengths & Weaknesses • Seek performance • Mainweakness • Avg magnetic disk: 30msec (some < 9 msec) • Avg on CD-ROM: 500msec (some < 105 msec) • Cantake much longer than average • Data transfer rate • Much betternow than book suggests • Still, slower than disks

  12. Storage capacity • Anadvantage • 700MB common • Read-only Access • Did have advantage of no changes • Not have to update file structures • Could optimize indexes • Advantagenolongerexists!

  13. Storageas a Hierarchy Registers, memory, RAM disk, disk cache Magnetic disk, Optical disks, mass storage Removable magnetic disk, Tapes According to Text

  14. For more of the hardware aspects of disks, tapes, CD, DVD, etc. see: • The PC Technology Guide • http://www.pctechguide.com/index.htm • Disctronics CD & DVD Technology • http://www.disctronics.co.uk/technology/

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