• Secondary storage device of resolution.
  • Main profit done tapes: disorganized entree vs. in order.
  • Data is keep and retrieved in units named disk blocks or pages.
  • Unlike RAM, instance to acquire a disk page varies depending upon position on saucer.
    • Therefore, qualifying spatial relation of pages on disc has core contact on DBMS performance!



Components of a Disk

  • The platters gyration (say, 90rps).
  • The arm machine is emotional in or out to place a leader on a sought after line. Tracks underneath heads net a container (imaginary!).
  • Only one lead reads/writes at any one incident.
  • Block size is a quadruplicate of sector volume (which is defined).

Accessing a Disk Page

  • Time to entree (read/write) a saucer block:
    • seek time (moving assemblage to defences round shape cranium on track)
    • rotational difficulty (waiting for stop up to spin underneath chief)
    • transfer juncture (actually vibratory data to/from round shape surface)


  • Seek incident and motion interruption rule.
    • Seek example varies from just about 1 to 20msec
    • Rotational hold varies from 0 to 10msec
    • Transfer charge is something like 1msec per 4KB leaf


  • Key to demean I/O cost: cut back movement/rotation delays! Hardware vs. package solutions?



Arranging Pages on Disk

  • 'Next' clog up concept:
    • blocks on selfsame track, followed by
    • blocks on one and the same cylinder, followed by
    • blocks on close container


  • Blocks in a data file should be placed sequentially on disc (by 'next'), to minimize motion and move glitch.

  • For a ongoing scan, pre-fetching various pages at a instance is a big win!

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