√2011-10-17-RAID - Storing Data: RAID...

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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 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?
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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 in a file should be arranged sequentially on disk (by `next’), to minimize seek and rotational delay. ± For a
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√2011-10-17-RAID - Storing Data: RAID...

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