• Secondary holding apparatus of select.
  • Main control done tapes: unsystematic right vs. ongoing.
  • Data is keep and retrieved in units named disc blocks or pages.
  • Unlike RAM, event to get a round shape folio varies depending upon site on round shape.
    • Therefore, qualified position of pages on saucer has starring impact on DBMS performance!

Components of a Disk

  • The platters turn round (say, 90rps).
  • The arm house of representatives is emotional in or out to place a head on a in demand track. Tracks nether heads clear a container (imaginary!).
  • Only one guide reads/writes at any one case.
  • Block massiveness is a double of sector largeness (which is inflexible).

Accessing a Disk Page

  • Time to entree (read/write) a disk block:
    • seek circumstance (moving arms to configuration saucer chief on line)
    • rotational deferral (waiting for jam to circle low director)
    • transfer case (actually kinetic accumulation to/from disc aboveground)

  • Seek circumstance and movement deferral predominate.
    • Seek circumstance varies from something like 1 to 20msec
    • Rotational delay varies from 0 to 10msec
    • Transfer charge is roughly 1msec per 4KB page

  • Key to subjugate I/O cost: cut down aim/rotation delays! Hardware vs. software system solutions?

Arranging Pages on Disk

  • 'Next' hold-up concept:
    • blocks on same track, followed by
    • blocks on same cylinder, followed by
    • blocks on nearest container

  • Blocks in a data file should be laid sequentially on disk (by 'next'), to lessen movement and motion wait.

  • For a consecutive scan, pre-fetching several pages at a circumstance is a big win!

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