After an introduction by FtfdO9

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         Disks

       Introduction
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                         Disks:
              summary / overview / abstract

       • The following gives an introduction to external memory
         for computers, focusing mainly on disks.
       • This presentation is preceded by a more general
         introduction to computer technology.
       • A separate but related presentation discusses optical disks
         like CD’s and DVD.
       • After an introduction, we focus on
         »Disk access time and data transfer rate
         »The capacity of various information carriers
         »Disk backup technologies
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                   Disks:
          overview of various types

       Magnetic disks   Optical disks

         Floppy disks   •Laserdisc
                        •WORM
                        •CD-ROM
                        •DVD
                        • ...


         Hard disks
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       !? Question !? Task !? Problem !?



                    Which disk types do you know
            besides magnetic floppy disks and hard disks,
                         and optical disks?
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                       Disks:
                  comparison of types

                                 =<
                               Floppy
                                          ;
                                          Hard     Optical Other
  Failure rate                 L High     J Low    J Low
  Disk capacity                L Low      J High   J High J High
  Storage cost per bit         L High     L High   J Low J Low
  Speed of
    data access and transfer   L Lowest   J High   L Low
  Exchangeability              J ++       L-       J+    L-
  Transportability             J+         L-       J+    J+
  Risk of disk crashes         J Low      L High   J Low
**--                                                                        6

                  Disks:
       decreasing prices: 1970-1995


                        Source cited in 1997:
                        http://community.bellcore.com/lesk/ages/ages.html




        ;
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                Disks:
       decreasing prices: 1995-…




                               ;
**--                                              8

       !? Question !? Task !? Problem !?


                  What are the consequences
                 of the decreasing disk prices?
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                               Disks:
                              formats

                         =<        ;
       Physical format              Hardware

       Logical format               Operating system

       Applications formats         Application programs
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            Physical and logical organization
                       of disk files

       The logical view of the user is translated by the operating


                                   ;
        system to the physical level.

                          =<
                         User
                           
                    Operating system Logical view
                                          
                                     Physical view/structure/level
**--                                                         11

                Data storage on disk:
           concentric tracks versus 1 spiral

       Storage method       Usage          Access Capacity

       Concentric tracks    Magnetic disks + Fast   - Low

       One continuous spiral CDs           - Slow   + High
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       Constant Angular Velocity versus
          Constant Linear Velocity

        Disk rotation velocity Usage

   CAV Constant               Magnetic disks drives
                              (and newer, faster CD players)

   CLV Changes as head moves CD players
       along the spiral
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           Disks

       Disk access time
              and
       data transfer rate
**--                                                                    14

                        Disk access time:
                           definition

       • Access time
         »= the time to locate a particular piece of information on a
          disk
         »= seek time + settling time + rotational delay
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                         Disk access time:
                          various causes

       Seek time =
         the time required to position the head assembly at the
           desired radius
       Settling time =
         the time required for the head to settle into position at the
           desired radius
       Rotational delay =
         the time the disk must rotate to get the desired sector under
           the read head
**--                                                        16

                    Disk access times
                of various types of disks


                             ;
                     Hard disk               CD

       Access time   from about 8 to 50 ms   about 300 ms
**--                                                                   17

                            Disks:
                      data transfer rate

       Transfer rate
       = the speed at which the computer reads data from a disk


                                                          ;
       once the data is found (kB/s)

                 =<
                  Floppy       CD-ROM                    Hard
                  disk                                   disk

 Transfer rate about           150 (digital audio, 1x)   3 000 up to
               30 kB/s     <   up to 8 000 kB/s (40x)    30 000 kB/s
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             Disks

              Capacity
       of information carriers
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                      Capacity of
                 information carriers

       Paper sheet A4                             2 Kbytes

       Floppy disks
          5.25" ss-dd                            180 Kbytes
          5.25" ds-dd                            360 Kbytes
          5.25" ds-hd                          1 200 Kbytes
          3.5" ds-dd                             720 Kbytes
          3.5" ds-hd                           1 440 Kbytes
       Hard disks on PCs   from 10 000 to 20 000 000 Kbytes

       Compact Disc                         600 000 Kbytes
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               Capacity of floppy disks:
                      evolution

       3000                                                                       2880


       2500

       2000

                                                1 4 4 01 4 4 01 4 4 01 4 4 01 4 4 0
       1500
                                  1 2 0 01 2 0 0

       1000

        500             360 360
              180 180

          0
              1981      1983      1985          1987         1989           1991
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            Evolution of disk capacities:
           advantages and disadvantages

   • Floppy disks + Hard disks
       J Fast evolution: increasing capacity
       L Incompatibilities between floppy disk and disk hardware


   • Compact disc
       L Stable from 1985: no increase of capacity
       J High compatibility with CD and DVD drives
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               Disks

       Disk backup technologies
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                Disk backup technologies:
                     various systems

       • Tape drives
       • Removable drives
       • Recordable CD (not rewritable)
       • Recordable and rewritable CD
       • Extra hard disk
       • DVD-RAM
       • Network drives
**--                                              24

                  Disk backup technologies:
                         tape drives

       • +Pros:
         »inexpensive hardware
         »low media cost
         »large capacity

       • -Cons:
         »slow
         »serial storage; no fast random access
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                  Disk backup technologies:
                      removable drives

       • +Pros:
         »inexpensive hardware
         »random access storage

       • -Cons:
         »limited to 2 GB (in 1999)
**--                                                  26

               Disk backup technologies:
             recordable CD (not rewritable)

       • Named CD-R
       • The files are not erasable; not rewritable
       • +Pros:
         »inexpensive; low media cost
         »random access storage
         »disks can be read by most CD-ROM drives
       • -Cons:
         »limited to 680 MB
         »Not erasable / rewritable / reusable
**--                                                    27

                Disk backup technologies:
              recordable and rewritable CD

       • Named CD-RW
       • The files are erasable; rewritable
       • +Pros:
         »inexpensive; low media cost
          (but more expensive than CD-R)
         »random access storage
       • -Cons:
         »limited to 680 MB
         »disks can NOT be read by most CD-ROM drives
**--                                                28

                  Disk backup technologies:
                       extra hard disk

       • +Pros:
         »fast
         »random access storage

       • -Cons:
         »expensive
         »not removable / cannot be kept off-site
**--                                          29

                  Disk backup technologies:
                         DVD-RAM

       • +Pros:
         »random access storage
         »large capacity

       • -Cons:
         »drives may be expensive

								
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