Chapter 3 Data Storage and Access Methods

1. Chapter 3Data Storage and Access Methods Title: Operating System Support for Database Management Author: Michael Stonebraker Pages: 217—223

2. Problem Definition • Apparent disconnect between DBMS performance goals and operating system design and implementation. • Services provided by OS are inadequate and sub-optimal. • Paper evaluates the following services: • Buffer pool management • File system • Interprocess communication • Consistency control • Paged virtual memory

3. Contributions • Demonstrates OS services are too slow or inappropriate for DBMS tasks. • Attempts to make OS designers aware of and more sensitive to DBMS needs.

4. Key Concepts • Buffer Pool Management • OS has a fixed buffer pool that handles all I/O • UNIX uses LRU replacement strategy, which may not be ideal for a DBMS • Large performance overhead to pull a block into the buffer. Approx. 5000 instructions for 512 bytes • No good prefetch strategy. • UNIX does not implement a selected force out buffer manager where the DBMS can dictate the order of the commits

5. Key Concepts • The File System • UNIX implements its file system as character arrays and forces the DBMS to implement its own higher level objects. • Tree Structured File Systems • UNIX implements 2 service using trees • Keeping track of blocks in a given file • Hierarchical directory structure • DBMS adds a third tree to support keyed access • One tree with all 3 kinds of information is more efficient.

6. Key Concepts • Scheduling Process Management and Interprocess Communication • Performance • Task switches are inevitable • Processes have a great deal of state information making task switches expensive • Critical Sections • Buffer pool is a shared data segment. • Problems arise if OS deschedules a DB process holding a lock on the buffer pool. • Server model • OS needs to provide a message facility for multiple processes to message a single process. • Server must do its own scheduling and multitasking.

7. Key Concepts • Consistency Control • Many Operating Systems can only place locks at the file level. • DBMS prefer finer granularity. • When DBMS implement its own buffer pool, crash recovery by the operating system would be impossible. • Paged Virtual Memory • Large files may not be able to be stored in memory • Binding chunks of the file into user space may incur a performance loss.

8. Validation • Content is mostly informational. • Based off previous papers and existing implementations of current systems. • Examples are cited primarily from the UNIX OS and the Ingres DBMS. • Issues could be biased and may not be common or applicable to all OS and DBMS combinations.

9. Assumptions • Presents the topic as one that is applicable to across a number of DBMS and OS • Author constrains his examples to UNIX and Ingres. • Paper was written in 1981. Operating Systems have advanced considerably since then. His points may no longer be applicable.

10. Changes if Rewritten Today • Increase the diversity of operating systems and DBMS • Add industry perspective. Are the problems Stonebraker presents really a problem for DBMS designers? • Quantify claims by providing statistical analysis of performance hits.