G sequen6al scans pages can be prefetched several

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Unformatted text preview: 6me B r Man anagement n a D MS Buffeuffer Magement in ia DBBMS Page Requests from Highe r Leve ls BUFFER POO L disk page free frame MAIN MEMORY DISK DB choice of frame dictated by replacement policy Data must be in RAM for DBMS to operate on it! •  Data must be in RAM for DBMS to operate on it! Table of <frame# , pageid> pairs is maintained. •  DaTable naf meframeNo, pRageNo> apdairs riks maintained tabase Ma oge < nt Systems 3ed , R. amakrishnan n J. Geh e 12 When a Page is Requested… •  If requested page is not in pool: –  Choose a frame for replacement –  If frame is dirty, write it to disk (“write on replacement”) –  Read requested page into chosen frame •  Pin the page and return its address •  If requests can be predicted (e.g., sequen6al scans), pages can be pre ­fetched several pages at a 6me –  Again, opportunity ignored in DavisDB for simplicity More on Buffer Management •  Requestor of page must unpin it, and indicate whether page has been modified –  Dirty bit is used for this •  Page in pool may be requested many 6mes –  A pin count (aka reference count) is used. A page is a candidate for replacement iff its pin count = 0 •  Concurrency control and recovery may entail addi6onal I/O when a frame is chosen for replacement. (Write ­Ahead Log protocol; more later…) –  No concurrency control or recovery in DavisDB Buffer Replacement Policy •  Frame is chosen for replacement by a replacement policy: –  Least ­recently ­used (LRU), Clock, MRU, etc –  DavisDB uses LRU •  Policy can have big impact on # of I/O's; depends on the access paDern •  Sequen3al flooding: nasty situa6on caused by LRU + repeated page scans –  # buffer frames < # pages in file means each page request causes an I/O. MRU much be\er in this situa6on (but not in all situa6ons, of course). DBMS vs. OS File System •  OS does disk space and buffer management; why not let the OS manage these tasks? •  Differences in OS support: portability issues •  Some technical limita6ons, e.g., files can...
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