A Secondary Storage: CD

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A Secondary StorageCD 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/

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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

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• 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!!

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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!

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• 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

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• 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

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• 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

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• 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
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• 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
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• (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

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CD-ROM Strengths Weaknesses

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

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• 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!

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Storage as a Hierarchy
Registers, memory, RAM disk, disk cache
Magnetic disk, Optical disks, mass storage
Removable magnetic disk, Tapes
According to Text
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• 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/