- 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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