1 / 9

Buffer Management

Buffer Management. Storage Technology: Topic 2. Effective Use of Main Memory. Memory is used as a cache for data on disk Issue: Granularity? Issue: Updates? Issue: Replacement? Issue: Efficient lookup? Issue: Concurrency?. Buffer Management in a DBMS. Page Requests from Higher Levels.

agatha
Télécharger la présentation

Buffer Management

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Buffer Management Storage Technology: Topic 2

  2. Effective Use of Main Memory • Memory is used as a cache for data on disk • Issue: Granularity? • Issue: Updates? • Issue: Replacement? • Issue: Efficient lookup? • Issue: Concurrency?

  3. Buffer Management in a DBMS Page Requests from Higher Levels • 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 DB

  4. When a Page is Requested ... • If the page is in the BP, go to 4 • Why would this happen? can it be encouraged? • Choose a frame to read into. • How does one do this? • Read page into chosen frame. • Pin the page and return to caller. • To avoid thrashing • The caller eventually unpins the page • Has the page been modified?

  5. Picking a Frame • If there is a free frame, use it • Use the page replacement policy to find a victim frame. • If victim frame is dirty, write it to disk (look at the next slide -- Update policy)

  6. Buffer Manager Details • Granularity : disk page size (4K, 8K) • Updates : “write-back” not “write-through” • Efficient lookup : hash table mapping pages to frames (hash function applied to what?) • Concurrency : what if several pages are requested at the same time? • What if the same page is requested multiply?

  7. Buffer Replacement Policy • Optimal policy needs to know future accesses. • At each stage, the victim should be the page used furthest into the future. • Practical repl. policies use a crystal ball. • Driven by knowledge of application behavior. • Least-recently-used (LRU), Clock, MRU, etc. • Pre-fetch, delayed-write, and other smarts.

  8. DBMS vs. OS File System OS does disk space & buffer mgmt: why not let OS manage these tasks? • Differences in OS support: portability issues • Some limitations, e.g., files can’t span disks. • Buffer management in DBMS requires: • pinning a page in buffer pool, forcing a page to disk (important for implementing CC & recovery) • adjust replacement policy and pre-fetch pages based on access patterns in typical DB operations

  9. Summary • Buffer manager brings pages into RAM. • Page stays in RAM until released by requestor. • Written to disk when frame chosen for replacement (which is sometime after requestor releases the page). • Choice of frame to replace based on replacement policy. • Tries to pre-fetch several pages at a time. • DBMS vs. OS File Support • DBMS needs features not found in many OS’s,

More Related