lec04.468

Lec04.468 - Spring 2008 CSC 468 DBMS Implementation Alexander Dekhtyar Storing Relational Data on Disk Disk Blocks Recall disk access operations

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. . Spring 2008 CSC 468: DBMS Implementation Alexander Dekhtyar . . Storing Relational Data on Disk Disk Blocks Recall : disk access operations are more expensive than data processing in main memory; an individual Read or Write command issued to a disk controller can re- trieve information stored in a number of consecutive disk sectors . To ensure that data stored in databases is retrieved efficiently, DBMS choose the following data storage approach: A block or disk page of a specific size is selected. The size of a block cannot be too large — this may lead to wasted space, but should not be too small — this will increase the number of disk access operations. Each relational table, and all supplemental index structures are stored as collections of blocks/pages on disk . Standard block sizes are 2, 4, 8, 16Kb. Larger blocks are used less often. Storing Relational Data in Disk blocks. Note, the the solution proposed below is by far, not the only possible. There may be a lot of different ways to store relational data on disk, e.g., grouping attributes together, using only index structures, storing data from different tables in the same file, etc. .. As we discuss the traditional storage techniques, it will become apparent why they are used and are considered efficient. 1
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Header Data Data Data Data Data Data Data An overview of a Relational Table File structure Figure 1: Database files on disk. Each relational table is stored on disk as a single file, broken into a sequence of disk pages. Individual blocks may be located in different places on disk (dif- ferent surfaces, tracks, cylinders, etc. . . ), but all content within a single page is a consequtive seqence of bytes. A
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This note was uploaded on 05/19/2008 for the course CSC 468 taught by Professor Dehktyar during the Spring '08 term at Cal Poly.

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Lec04.468 - Spring 2008 CSC 468 DBMS Implementation Alexander Dekhtyar Storing Relational Data on Disk Disk Blocks Recall disk access operations

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