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Unformatted text preview: Storing Data: RAID Database Management System, R. Ramakrishnan and J. Gehrke 1 Components of a Disk The platters spin (say, 10K rpm). Spindle The arm assembly is moved in or out to position a head on a desired track. Tracks under heads make Disk head Tracks Sector Database Management System, R. Ramakrishnan and J. Gehrke 2 Platters Tracks under heads make a cylinder (imaginary!). Arm movement Arm assembly Only one head reads/writes at any one time. Block size is a multiple of sector size (which is fixed). Accessing a Disk Page Time to access (read/write) a disk block: – seek time (moving arms to position disk head on track) – rotational delay (waiting for block to rotate under head) – transfer time (actually moving data to/from disk surface) Database Management System, R. Ramakrishnan and J. Gehrke 3 Seek time and rotational delay dominate. – Seek time varies from about 1 to 10msec – Rotational delay varies from 0 to 5msec – Transfer rate is about 0.1-0.5msec per 4KB page Key to lower I/O cost: reduce seek/rotation delays! Hardware vs. software solutions? Arranging Pages on Disk ` Next ’ block concept: – blocks on same track, followed by – blocks on same cylinder, followed by – blocks on adjacent cylinder Database Management System, R. Ramakrishnan and J. Gehrke 4 blocks on adjacent cylinder Blocks in a file should be arranged sequentially on disk (by `next’), to minimize seek and rotational delay....
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- Fall '07
- Database management system, RAID, R. Ramakrishnan, J. Gehrke, Rotating disc computer storage media