[CMPT 454] Week 1_3

June·2021년 1월 17일

CMPT 454

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Buffer Replacement Policy

  • Frame is chosen for replacement by a replacement policy
    • Ideas: choose frame that least likely will be used in future, e.g., FIFO, least-recently-used(LRU), most-recently-used(MRU), etc.
  • Policy can have impact on # of I/O's; depends on access pattern
  • Sequential flooding: Nasty situation caused by LRU + repeated sequential scans when # frames < # pages in file.

    Worst case. should buffer out when read page.

DBMS vs. OS File System

OS does disk space & buffer mgmt too: why not let OS manage these tasks?

  • DBMS wants to support multiple OS platforms
  • OS files can't span disks
  • DBMS requires to
    • pin a page in buffer pool, force a page to disk (for implementing CC & recovery)
    • adjust replacement policy and pre-fetch pages based on access patterns

How to store fields, records, pages, and files?

Records Formats: Fixed Length

Records Formats: Variable Length

Page Formats: Fixed Length Records

왼쪽의 경우에 한 record를 삭제할 경우, 다 밀려 올라간다. 따라서 rid도 바뀐다. 비효율적이다.
오른쪽의 경우에는 비트맵으로 빈 슬롯인지 아닌지 관리한다. 이 경우 rid가 바뀌지 않는다.

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