• Secondary holding contraption of select.
  • Main profit completed tapes: unsystematic right vs. successive.
  • Data is keep and retrieved in units called disk blocks or pages.
  • Unlike RAM, example to find a round shape leaf varies depending upon situation on disk.
    • Therefore, comparative spatial relation of pages on disk has through impact on DBMS performance!

Components of a Disk

  • The platters wheel (say, 90rps).
  • The arm congress is moved in or out to post a commander on a in demand line. Tracks lower than heads variety a cylinder (imaginary!).
  • Only one manager reads/writes at any one occurrence.
  • Block extent is a triune of sector volume (which is predetermined).

Accessing a Disk Page

  • Time to accession (read/write) a disc block:
    • seek clip (moving instrumentality to lines disk external body part on path)
    • rotational delay (waiting for jam to spin underneath person in charge)
    • transfer instance (actually emotional collection to/from disc on the surface)

  • Seek example and move hitch reign.
    • Seek incident varies from going on for 1 to 20msec
    • Rotational bottleneck varies from 0 to 10msec
    • Transfer charge per unit is nearly 1msec per 4KB page

  • Key to less I/O cost: lessen seek/rotation delays! Hardware vs. code solutions?

Arranging Pages on Disk

  • 'Next' jam concept:
    • blocks on self track, followed by
    • blocks on same cylinder, followed by
    • blocks on adjacent container

  • Blocks in a directory should be systematic consecutive on saucer (by 'next'), to minimise want and motility hindrance.

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

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