koeeoj的部落格

• 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!
  
  

• 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).

• 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?

• '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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