6. Processing Overview (Buffer Manager)

• Decides when to move data between disk and RAM
• Reduce data movements using heuristics
• Buffer manager manages “buffer pool”
  • Buffer pool: main memory reserved for DBMS
  • frames: page-sized slots
  • Stores meta-data about each slot

Frame Properties

• Page ID:
  • Find page, then frame
• Pin count: # of processes using the page
  • Evict page if pin count reaches zero
• Dirty bit: memory data != storage data (updated data)
  • Write page to disk before evicting it

Don’t rely on OS

• OS uses virtual memory caches pages
• We want a separate buffer manager
  • DBMS knows its access patterns ahead of time
    • Smarter replacements
  • DBMS must control page writes for safety guarantees

Processing Page Requests (buffer manager)

• Cache Hit (requested page cached)
  • Increase pin count & return page address
• Cache Miss (requested page not cached)
  • Choose frame for storage data (replacement policy)
  • If frame contains dirty page: write to disk
    • Increase pin count and return page address
  • Read requested page from disk and store in frame, Set pint count to 0

LRU

• Replace page required farthest in the future
  • Reduce expensive cache misses
• However: difficult to predict that in general
• Heuristic: remove least recently used page (LRU)
  • remove use page which did not use for long time

Sequential Flooding (when LRU is no good)

• buffer pool’s contents are corrupted due to a sequential scan.
• Since sequential scans read every page, the timestamps of pages read may not reflect which pages we actually want.
  • the most recently used page is actually the most unneeded page.
• DBMS often have particular access patterns
  • e.g. Scanning pages in round robin mode (circular or repeated sequence)
• Least recently used page is used again soonest
  • LRU policy gets sub-optimal (inefficient)