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

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

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

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

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

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

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

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

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

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