- Secondary storage device of resolution.
- Main ascendancy done tapes: impulsive accession vs. successive.
- Data is keep and retrieved in units called round shape blocks or pages.
- Unlike RAM, time to get a round shape folio varies depending upon situation on disc.
- Therefore, relative arrangement of pages on saucer has major impact on DBMS performance!
Components of a Disk
- The platters swivel (say, 90rps).
- The arm house is stirred in or out to part a pave the way on a sought after path. Tracks under heads variety a cylinder (imaginary!).
- Only one commander reads/writes at any one instance.
- Block sized is a fourfold of plane figure proportions (which is defined).
Accessing a Disk Page
- Time to entree (read/write) a round shape block:
- seek clip (moving accumulation to position saucer head on track)
- rotational time lag (waiting for blockage to gyrate lower than leader)
- transfer juncture (actually wiggling data to/from saucer seeming)
- Seek juncture and move break prevail.
- Seek clip varies from in the region of 1 to 20msec
- Rotational suspension varies from 0 to 10msec
- Transfer charge per unit is almost 1msec per 4KB leaf
- Key to demean I/O cost: slim down movement/rotation delays! Hardware vs. code solutions?
Arranging Pages on Disk
- 'Next' obstruct concept:
- blocks on self track, followed by
- blocks on aforesaid cylinder, followed by
- blocks on bordering container
- Blocks in a database should be systematic sequentially on saucer (by 'next'), to decrease seek and motility interruption.
- For a serialized scan, pre-fetching individual pages at a occurrence is a big win!
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