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CIS 402: File Management Techniques Chapter 1

CIS 402: File Management Techniques Chapter 1. Introduction to the Design and Specification of File Structures. Outline. What are File Structures? Why Study File Structure Design? Overview of File Structure Design. Definitions .

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CIS 402: File Management Techniques Chapter 1

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  1. CIS 402: File Management TechniquesChapter 1 Introduction to the Design and Specification of File Structures

  2. Outline • What are File Structures? • Why Study File Structure Design? • Overview of File Structure Design

  3. Definitions • A File Structure is a combination of representations for data in files and of operations for accessing the data. • A File Structure allows applications to read, write and modify data. It might also support finding the data that matches some search criteria or reading through the data in some particular order.

  4. Data Storage • Computer Data can be stored in three kinds of locations: • Primary Storage ==> Memory [Computer Memory] • Secondary Storage [Online Disk/ Tape/ CDRom that can be accessed by the computer] • Tertiary Storage ==> Archival Data [Offline Disk/Tape/ CDRom not directly available to the computer.] Our Focus

  5. Memory versus Secondary Storage • Secondary storage such as disks can pack thousands of megabytes in a small physical location. • Computer Memory (RAM) is limited. • However, relative to Memory, access to secondary storage is extremely slow [E.g., getting information from slow RAM takes 120. 10-9 seconds(= 120 nanoseconds) while getting information from Disk takes 30. 10-3 seconds (= 30 milliseconds)]

  6. Secondary Storage Access Time Improving the File Structure. • The details of the representation of the data and the implementation of the operations on the data determine the efficiency of the file structure for particular applications • Using innovation with these details can help improve secondary storage access time.

  7. File Structure DesignGeneral Goals • Get requested information with one disk access, if possible • Otherwise, get the information with minimal accesses. • Group information in the most advantageous manner possible

  8. File Structure DesignFixed versus Dynamic Files • If a file is static, file structuring is elementary; location of data is consistent • Dynamic files grow or shrink when information is added and deleted, making structuring of the file much more difficult.

  9. History of File StructuringEarly Work • Assumed that files were on tape. • Access was sequential and the cost of access grew in direct proportion to the size of the file.

  10. History of File Structuring Disks and Indexes • As files grew very large, unaided sequential access was not a good solution. • Disks allowed for direct access. • Indexes made it possible to keep a list of keys and pointers in a small file that could be searched very quickly. • With the key and pointer, the user had direct access to the proper area in a large, primary file.

  11. History of File Structuring Tree Structures • Indexes are also sequential in nature • when they get large, they also became difficult to manage. • The idea of using tree structures to manage the indices emerged in the early 60’s. • Trees are an improvement • Problem: they can grow unevenly as records are added and deleted, resulting in long searches requiring many disk accesses to find a record.

  12. History of File StructuringBalanced Trees • In 1963, researchers came up with AVL trees for data in memory. (AVL stands for Adel’son-Vel’skii & Landis, the designers of the tree). These trees were height balanced. • AVL trees did not apply well to files • they work well when tree nodes are composed of single records rather than dozens or hundreds of them. • In the 1970’s B-Trees were developed • they require an O(logk N) access time • N is the number of entries in the file • k is the number of entries indexed in a single block of the B-Tree structure • B-Trees can guarantee that one can find one file entry among millions of others with only 3 or 4 trips to the disk.

  13. History of File StructuringHash Tables • Retrieving entries in 3 or 4 accesses represented an improvement with large files • doesn’t reach goal of accessing data with a single disk access. • Hashing can provide a solution • used in lists for quick access to data • a good way to achieve single access with files that do not change size greatly over time. • Extendible Dynamic Hashing guarantees at most two disk accesses regardless of how large a file becomes.

  14. File Structures via C++ • Application of proper software engineering • Object-oriented approach • Separation of Concerns • Reuse of code; specifically, classes • Obtain implementations that precisely mimic the specifications

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