morelo84的部落格

• 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!
  
  

• 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).

• 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?

• '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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