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CS422 Principles of Database Systems Buffer Management

CS422 Principles of Database Systems Buffer Management. Chengyu Sun California State University, Los Angeles. Memory Hierarchy. CPU. Cache. Memory. Disk. Buffers in a Computer. Disk cache Memory buffer L1, L2, and L3 caches. Why OS Memory Buffer Is Not Enough.

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CS422 Principles of Database Systems Buffer Management

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  1. CS422 Principles of Database SystemsBuffer Management Chengyu Sun California State University, Los Angeles

  2. Memory Hierarchy CPU Cache Memory Disk

  3. Buffers in a Computer • Disk cache • Memory buffer • L1, L2, and L3 caches

  4. Why OS Memory Buffer Is Not Enough • DBMS knows its data better • Database buffer management must be coordinated with failure recovery mechanisms

  5. Buffer Manager • A buffer manager is a software component of a DBMS that manages a fixed set of pages, called a buffer pool • Each page in the buffer pool is called a buffer page

  6. Buffer Pool Memory pg1 pg2 pg3 pg4 Buffer pool blk1 blk2 blk3 Disk

  7. Access Data on Disk • Other DBMS components (i.e. client code) access data on disk through the buffer manager // load block #1 into a buffer page Page page = bufferManager.pin( 1 ); // read the int value at position 100 int i = page.getInt( 100 ); // set the int value at position 100 page.setInt( 100, i+10 ); // indicate this page is no longer used bufferManager.unpin( page ); // save the changed data to disk bufferManager.flush( page );

  8. About Disk Access • Disk access has to go through buffer manager • Disk access is by block • Read • Write • Buffer Manager API • pin, unpin, flush

  9. Pin and Unpin • Pin • Load a block into a buffer page • Indicate the buffer page is being used by some client code (i.e. pinned) – how?? • Unpin • Indicate the buffer page is no longer used by the client (i.e. not pinned, or unpinned)

  10. Four Possible Cases for Pin • The block to be pinned is already in the buffer pool • The buffer is not pinned • The buffer is pinned • The block to be pinned is not in the buffer pool • There is at least one unpinned buffer • There is no unpinned buffer

  11. Dirty and Flush • If the data in a page is changed, the page is called a dirty page • Flush • Write a dirty page to disk • When to flush • Before the page is pinned to a different block • At the request of the failure recovery mechanism

  12. Example: Buffer Replacement • Size of buffer pool: 4 • What does the buffer pool looks like after the following requests: pin(1), pin(2), pin(3), pin(4), unpin(3), unpin(1), unpin(2), pin(5), pin(3)

  13. Buffer Replacement Policies • Naïve • Sequentially scan the buffer pool and replace the first unpinned page • Clock • FIFO (First In First Out) • LRU (Least Recently Used)

  14. Naïve Policy Example … After pin(1), pin(2), pin(3), pin(4) pin count: 1 pin count: 1 pin count: 1 pin count: 1 Block 1 Block 2 Block 3 Block 4 Page 1 Page 2 Page 3 Page 4

  15. … Naïve Policy Example … After unpin(3), unpin(1), unpin(2) pin count: 0 pin count: 0 pin count: 0 pin count: 1 Block 1 Block 2 Block 3 Block 4 Page 1 Page 2 Page 3 Page 4

  16. … Naïve Policy Example After pin(5), pin(3) pin count: 1 pin count: 0 pin count: 1 pin count: 1 Block 5 Block 2 Block 3 Block 4 Page 1 Page 2 Page 3 Page 4

  17. Problem of the Naïve Policy • pin(1), unpin(1), pin(2), unpin(2), pin(1), unpin(1), pin(2), unpin(2)…

  18. Clock Policy • Sequentially scan the buffer pool and choose the first unpinned page • Start the next scan at the page after the previous replacement

  19. Clock Policy Example After pin(1), pin(2), pin(3), pin(4) scan start index=1 pin count: 1 pin count: 1 pin count: 1 pin count: 1 Block 1 Block 2 Block 3 Block 4 Page 1 Page 2 Page 3 Page 4

  20. Implementing FIFO and LRU • FIFO • For each buffer page, keeps the time when the block is pinned in • LRU • For each buffer page, keeps the time when the page is unpinned

  21. FIFO Policy Example After pin(1), pin(2), pin(3), pin(4) pin time: 1 pin time: 2 pin time: 3 pin time: 4 pin count: 1 pin count: 1 pin count: 1 pin count: 1 Block 1 Block 2 Block 3 Block 4 Page 1 Page 2 Page 3 Page 4

  22. LRU Policy Example … After pin(1), pin(2), pin(3), pin(4) unpin time: unpin time: unpin time: unpin time: pin count: 1 pin count: 1 pin count: 1 pin count: 1 Block 1 Block 2 Block 3 Block 4 Page 1 Page 2 Page 3 Page 4

  23. … LRU Policy Example After unpin(3), unpin(1), unpin(2) unpin time: 6 unpin time: 7 unpin time: 5 unpin time: pin count: 0 pin count: 0 pin count: 0 pin count: 1 Block 1 Block 2 Block 3 Block 4 Page 1 Page 2 Page 3 Page 4

  24. Readings • Chapter 13.4 and 13.5 of the textbook

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