• Secondary storage device of resolution.
  • Main ascendancy done tapes: impulsive accession vs. successive.
  • Data is keep and retrieved in units called round shape blocks or pages.
  • Unlike RAM, time to get a round shape folio varies depending upon situation on disc.
    • Therefore, relative arrangement of pages on saucer has major impact on DBMS performance!

Components of a Disk

  • The platters swivel (say, 90rps).
  • The arm house is stirred in or out to part a pave the way on a sought after path. Tracks under heads variety a cylinder (imaginary!).
  • Only one commander reads/writes at any one instance.
  • Block sized is a fourfold of plane figure proportions (which is defined).

Accessing a Disk Page

  • Time to entree (read/write) a round shape block:
    • seek clip (moving accumulation to position saucer head on track)
    • rotational time lag (waiting for blockage to gyrate lower than leader)
    • transfer juncture (actually wiggling data to/from saucer seeming)

  • Seek juncture and move break prevail.
    • Seek clip varies from in the region of 1 to 20msec
    • Rotational suspension varies from 0 to 10msec
    • Transfer charge per unit is almost 1msec per 4KB leaf

  • Key to demean I/O cost: slim down movement/rotation delays! Hardware vs. code solutions?

Arranging Pages on Disk

  • 'Next' obstruct concept:
    • blocks on self track, followed by
    • blocks on aforesaid cylinder, followed by
    • blocks on bordering container

  • Blocks in a database should be systematic sequentially on saucer (by 'next'), to decrease seek and motility interruption.

  • For a serialized scan, pre-fetching individual pages at a occurrence is a big win!

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