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!! • Recordable CDs are now common • Cheaper • Faster • DVD is also available for optical disks • Digital Video Disc • Digital Versatile Disk

CD challenges • Strengths • High storage 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!!

Physical Organization of CDs • Started as medium for audio • Designed for music • High capacity • Moderate transfer rate • Not designed for fast, random data access • No concerns for fast seek performance • Good random access performance must come from file structure design!

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

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: large storage versus seek time • CAV would have almost half present capacity • CLV seeking uses trial-and-error speed control

Speed notes • Speeds given in relation to original audio CD speeds • “single-speed” – 150 KBps • Other speeds are multiples of this • 48-speed or 48x means 48 times faster • Like Hertz commercial – “not exactly”! • About 19x on innermost tracks • Only 48x on the outer tracks • Exception: • TrueX technology from Zen Research (Kenwood) • 52x gets 45x-52x across the entire disk

CD-RW • CD-ReWritable • User can record files, delete files, etc. • Recording layer is different • Crystalline compound of silver, indium, antimony, tellurium • Heat to one temp & cool  reflective • Heat to higher temp & 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

DVD • Digital Video (or Versatile) Disc • Same size as a CD • 7-fold increase in capacity • Largely due to improvements in technology • (1) Closer tracks; smaller pits CD DVD

(2) More than one layer of data is possible • (3) Can use double-sided disks • (4) More efficient structure of data • Error correction code is better • Needs less bits  more room for data

CD-ROM Strengths & Weaknesses • Seek performance • Main weakness • Avg magnetic disk: 30 msec (some < 9 msec) • Avg on CD-ROM: 500 msec (some < 105 msec) • Can take much longer than average • Data transfer rate • Much better now than book suggests • Still, slower than disks

Storage capacity • An advantage • 700 MB common • Read-only Access • Did have advantage of no changes • Not have to update file structures • Could optimize indexes • Advantage no longer exists!

Asymmetric writing and reading • Write once; read many times • This is no longer true! • Intelligent file structures is no longer a one time investment

Storage as a Hierarchy Registers, memory, RAM disk, disk cache Magnetic disk, tape, mass storage Removable magnetic disk, optical disks, tapes According to Text

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/

A Secondary Storage: CD – ROM