• Secondary holding machinery of judgment.
  • Main dominance ended tapes: erratic admittance vs. sequential.
  • Data is keep and retrieved in units called disc blocks or pages.
  • Unlike RAM, circumstance to recover a round shape page varies depending upon location on disc.
    • Therefore, qualified spatial relation of pages on saucer has prima striking on DBMS performance!



Components of a Disk

  • The platters pivot (say, 90rps).
  • The arm parliament is moved in or out to defences a commander on a desirable track. Tracks underneath heads put together a cylinder (imaginary!).
  • Only one caput reads/writes at any one incident.
  • Block magnitude is a sextuple of two-dimensional figure vastness (which is steady).

Accessing a Disk Page

  • Time to accession (read/write) a round shape block:
    • seek time (moving weapons system to situation saucer principal on path)
    • rotational rearrangement (waiting for artifact to rotate underneath come first)
    • transfer incident (actually emotional notes to/from saucer plane)


  • Seek incident and motility intermission prevail.
    • Seek case varies from just about 1 to 20msec
    • Rotational suspension varies from 0 to 10msec
    • Transfer rate is roughly speaking 1msec per 4KB leaf


  • Key to belittle I/O cost: cut hope/rotation delays! Hardware vs. software solutions?



Arranging Pages on Disk

  • 'Next' log jam concept:
    • blocks on very track, followed by
    • blocks on said cylinder, followed by
    • blocks on next container


  • Blocks in a folder should be placed sequentially on round shape (by 'next'), to minimize aim and movement hindrance.

  • For a successive scan, pre-fetching respective pages at a juncture is a big win!

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