MidtermAns

MidtermAns - - 1 - University of Waterloo School of...

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Unformatted text preview: - 1 - University of Waterloo School of Computer Science CS348 Spring 2009 Midterm Test Time allowed: 60 minutes This is a closed-book test Do not open this test paper until you are instructed to do so. Please answer all questions in the space provided. Student Number:___________________ Student Name:____________________________ Q1: /25 Q2: /25 Q3: /25 Q4: /25 ----------------- Total: /100 - 2 - 25% 1. 9%(a) What do we mean by a system achieves a high level of logical data independence. Briefly explain why the 3-level approach of data management achieves the objective of high level of logical data independence? A system achieves a high level of logical d.i. if changes in logical aspects of data, such as change the definition of record type or a relation scheme by adding/deleting attributes, or adding/deleting record type or relation schemes in the conceptual schema, will have a minimal effect on application programs. At the conceptual level, only logical aspects of data are specified and since an external schema is defined on a minimal subset of objects in the conceptual schema that is required by an application program. Thus, an application program is completely insulated from those data that it cannot see or use. As a result, changes in the logical aspect of data are completely insulated from an application program if those changes are not seen or used by the application program. As a result, such an architecture achieves a high level of logical data independence. 5%(b) What is the ordering of blocks that is imposed on a disk pack in a disk drive? What are the advantages of such an ordering? A block address is of the form <cyl#, track#, block#>. The blocks in a disk drive are ordered according to the cylinder numbers (say, cylinder 1, 2.., n). Within each cylinder, blocks are ordered according to the track or surface number. Within a track on a surface, the blocks are ordered by the block number. In a disk-storage system, the most costly components in accessing a block are the seek time and the rotational delay. The advantage of such an ordering is to allow a file of records to be accessed efficiently. Such an organization minimizes the seek time and rotational delay if records in a file are stored and accessed sequentially. file are stored and accessed sequentially....
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MidtermAns - - 1 - University of Waterloo School of...

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