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Data Structures-B trees

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B-Trees CSE 373 Data Structures CSE 373 -- AU 2004 -- B-Trees 1 B-Trees Considerations for disk-based storage systems. Indexed Sequential Access Method (ISAM) m-way search trees B-trees CSE 373 -- AU 2004 -- B-Trees 2 Data Layout on Disk • Track: one ring • Sector: one pie-shaped piece. • Block: intersection of a track and a sector. CSE 373 -- AU 2004 -- B-Trees 3 Disk Block Access Time Seek time = maximum of Time for the disk head to move to the correct track. Time for the beginning of the correct sector to spin round to the head. (Some authors use “latency” as the term for this component, or they use latency to refer to all of what we are calling seek time.) Transfer time = Time to read or write the data. (Approximately the time for the sector to spin by the head). For a 7200 RPM hard disk with 8 millisec seek time, average access time for a block is about 12 millisec. (see Albert Drayes and John Treder: http://www.tanstaafl-software.com/seektime.html) CSE 373 -- AU 2004 -- B-Trees 4 Considerations for Disk Based Dictionary Structures Use a disk-based method when the dictionary is too big to fit in RAM at once. Minimize the expected or worst-case number of disk accesses for the essential operations (put, get, remove). Keep space requirements reasonable -- O(n). Methods based on binary trees, such as AVL search trees, are not optimal for disk-based representations. The number of disk accesses can be greatly reduced by using m-way search trees. CSE 3 ...
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