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CS222P: Principles of Data Management Lecture #3 Buffer Manager , PAX

CS222P: Principles of Data Management Lecture #3 Buffer Manager , PAX. Instructor: Chen Li. DB. Next topic: Buffer Management. Page Requests from Higher Levels. Note: Project 1 ’ s PagedFileManager class would do the buffering inside if we were doing it…!.

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CS222P: Principles of Data Management Lecture #3 Buffer Manager , PAX

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  1. CS222P: Principles of Data ManagementLecture #3Buffer Manager,PAX Instructor: Chen Li

  2. DB Next topic: Buffer Management Page Requests from Higher Levels Note: Project 1’s PagedFileManager class would do the buffering inside if we were doing it…! • Data must be in RAM for DBMS to operate on it! • Table of <frame#, pageid> pairs is maintained. BUFFER POOL disk page free frame MAIN MEMORY DISK choice of frame dictated by replacement policy

  3. When a Page is Requested ... • If requested page is not in pool: • Choose a frame for replacement • If that frame is dirty, write it to disk • Read requested page into chosen frame • Pin the page and return its address * If requests can be predicted (e.g., sequential scans) pages can be prefetchedseveral pages at a time!

  4. More on Buffer Management • Requestor of page must unpin it, and indicate whether page has been modified, when done: • dirtybit used for the latter purpose • Page in pool may be requested many times • a pin count is used, and a page is a candidate for replacement iffpin count = 0. • CC & recovery may entail additional I/O when a frame is chosen for replacement. (Write-Ahead Log protocol; more in CS 223.)

  5. Buffer Replacement Policy • Frame is chosen for replacement using a replacement policy: • Least-recently-used (LRU), Clock, MRU, etc. • Policy can have big impact on # of I/O’s; depends on the access pattern. • Sequential flooding: Nasty situation caused by LRU + (repeated) sequential scans. • # buffer frames < # pages in file means each page request causes an I/O. MRU much better in this situation (but not in all situations, of course).

  6. PAX format Traditional Format PAX Format • PAX partitions each page into minipages based on fields • Good caching behaviors for “select fields from …”; • Compression • www.pdl.cmu.edu/PDL-FTP/Database/pax.pdf • Column store (e.g., Vertica)

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