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CS4432: Database Systems II

CS4432: Database Systems II. Lecture #3 Using the Disk, and Disk Optimizations. Professor Elke A. Rundensteiner. Thus far :. Hardware: Disks Architecture: Layers of Access Access Times and Abstractions. Optimizations (in controller or O.S.). Disk Scheduling Algorithms

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CS4432: Database Systems II

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  1. CS4432: Database Systems II Lecture #3 Using the Disk, and Disk Optimizations Professor Elke A. Rundensteiner lecture #3

  2. Thus far : Hardware: Disks Architecture: Layers of Access Access Times and Abstractions lecture #3

  3. Optimizations(in controller or O.S.) Disk Scheduling Algorithms e.g., elevator algorithm Larger Buffer Pre-fetch Disk Arrays Disk Cache CS 245 lecture #3 Notes 2 3

  4. One Simple Idea : Prefetching Problem: Have a File • Sequence of Blocks B1, B2 Have a Program • Process B1 • Process B2 • Process B3 ... lecture #3

  5. Single Buffer Solution (1) Read B1  Buffer (2) Process Data in Buffer (3) Read B2  Buffer (4) Process Data in Buffer ... lecture #3

  6. Say P = time to process/block R = time to read in 1 block n = # blocks Single buffer time = n(P+R) lecture #3

  7. Question: Could the DBMS know something about behavior of such future block accesses ? What if: If we knew more about the sequence of future block accesses, how could we do better ? lecture #3

  8. process process B C B A A A B C D E F G done done Idea : Double Buffering/Prefetching Memory: Disk: lecture #3

  9. Say P  R P = Processing time/block R = IO time/block n = # blocks What is processing time now? • Double buffering time = ? lecture #3

  10. Say P  R P = Processing time/block R = IO time/block n = # blocks • Double buffering time = R + nP • Single buffering time = n(R+P) lecture #3

  11. Disk Arrays RAIDs (various flavors) Block Striping Mirrored logically one disk lecture #3 Notes 2 11

  12. On Disk Cache P ... ... M C cache cache lecture #3 Notes 2 12

  13. Block Size Selection? • Question : Do we want Small or Big Block Sizes ? • Pros ? • Cons ? lecture #3

  14. Unfortunately... • Big Block  Read in more useless stuff! • and takes longer to read Block Size Selection? • Big Block  Amortize I/O Cost • For seek and rotational delays are reduced … lecture #3

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