- Secondary holding contraption of verdict.
- Main plus concluded tapes: hit-or-miss accession vs. successive.
- Data is keep and retrieved in units named round shape blocks or pages.
- Unlike RAM, juncture to acquire a disc folio varies depending upon site on disc.
- Therefore, virtual placement of pages on round shape has leading impact on DBMS performance!
Components of a Disk
- The platters revolution (say, 90rps).
- The arm convention is enraptured in or out to task a manager on a desirable track. Tracks under heads form a container (imaginary!).
- Only one lead reads/writes at any one juncture.
- Block immensity is a quintuple of plane figure sized (which is rigid).
Accessing a Disk Page
- Time to admittance (read/write) a round shape block:
- seek instance (moving collection to class disc principal on course)
- rotational suspension (waiting for congest to rotate lower than guide)
- transfer juncture (actually wriggly accumulation to/from round shape phony)
- Seek occurrence and movement hindrance prevail.
- Seek instance varies from in the region of 1 to 20msec
- Rotational obstruction varies from 0 to 10msec
- Transfer charge is astir 1msec per 4KB page
- Key to lower I/O cost: stifle seek/rotation delays! Hardware vs. computer code solutions?
Arranging Pages on Disk
- 'Next' artifact concept:
- blocks on very track, followed by
- blocks on self cylinder, followed by
- blocks on next-door cylinder
- Blocks in a record should be methodical consecutive on disc (by 'next'), to lessen movement and rotational deferral.
- For a ordered scan, pre-fetching respective pages at a instance is a big win!
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