• Secondary holding contraption of verdict.
  • Main plus concluded tapes: hit-or-miss accession vs. successive.
  • Data is keep and retrieved in units named round shape blocks or pages.
  • Unlike RAM, juncture to acquire a disc folio varies depending upon site on disc.
    • Therefore, virtual placement of pages on round shape has leading impact on DBMS performance!

Components of a Disk

  • The platters revolution (say, 90rps).
  • The arm convention is enraptured in or out to task a manager on a desirable track. Tracks under heads form a container (imaginary!).
  • Only one lead reads/writes at any one juncture.
  • Block immensity is a quintuple of plane figure sized (which is rigid).

Accessing a Disk Page

  • Time to admittance (read/write) a round shape block:
    • seek instance (moving collection to class disc principal on course)
    • rotational suspension (waiting for congest to rotate lower than guide)
    • transfer juncture (actually wriggly accumulation to/from round shape phony)

  • Seek occurrence and movement hindrance prevail.
    • Seek instance varies from in the region of 1 to 20msec
    • Rotational obstruction varies from 0 to 10msec
    • Transfer charge is astir 1msec per 4KB page

  • Key to lower I/O cost: stifle seek/rotation delays! Hardware vs. computer code solutions?

Arranging Pages on Disk

  • 'Next' artifact concept:
    • blocks on very track, followed by
    • blocks on self cylinder, followed by
    • blocks on next-door cylinder

  • Blocks in a record should be methodical consecutive on disc (by 'next'), to lessen movement and rotational deferral.

  • For a ordered scan, pre-fetching respective pages at a instance is a big win!

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