Chapter 3 Data Storage and Access Methods Title: Operating Systems Support for Database Management Author: Michael Stonebraker Pages: 217 – 223 Group 01: Esten Rye, DJ Oneil

Overview  Problem Definition  Motivation – Why is this Important?  Key concepts Buffer Pool Management Buffer Pool Management The File System The File System Process Scheduling, Management, Inter-process Communication Process Scheduling, Management, Inter-process Communication Consistency Control and Virtual Page File Consistency Control and Virtual Page File  Validation  Assumptions  Rewrite

Problem Statement  Given Operating System Services – buffer pool management, file system, scheduling, etc… Operating System Services – buffer pool management, file system, scheduling, etc… Database Management Functions – concurrency, persistence, performance, query, etc… Database Management Functions – concurrency, persistence, performance, query, etc…  Objective Examine applicability of OS services to the support of DBMS functions. Examine applicability of OS services to the support of DBMS functions.  Find Apparent disconnect between DBMS performance goals and OS design and implementation. Apparent disconnect between DBMS performance goals and OS design and implementation.  Constraints OS Services must as reliable as DBMS functions OS Services must as reliable as DBMS functions

Why is this Important?  Current Database Management Systems Usually provide their own versions of OS services. Usually provide their own versions of OS services. Make little use of those provided by the OS. Make little use of those provided by the OS.  Current Operating Systems Many services provided by the OS are too slow or inappropriate. Many services provided by the OS are too slow or inappropriate. Many OS designers are insensitive to DBMS needs. Many OS designers are insensitive to DBMS needs.

Buffer Pool Management  Blocks with a locality of reference remain in cache over repeated reads and writes.  Problems with OS-provided service Performance Overhead Performance Overhead LRU Replacement performs marginally in a database environment. LRU Replacement performs marginally in a database environment. Does not guarantee the intentions list and commit flag will be pushed to disk in the proper order. Does not guarantee the intentions list and commit flag will be pushed to disk in the proper order.

The File System Two Approaches  Character arrays of dynamically varying size. File system provided by UNIX File system provided by UNIX DBMS can provide whatever abstraction it needs on top of this. DBMS can provide whatever abstraction it needs on top of this.  Record management system inside the OS. DBMS wants this approach. DBMS wants this approach. Not always efficient when constructed on top of a character array object. Not always efficient when constructed on top of a character array object.

The File System Problems and Limitations  Character Array is not a useful object to a DBMS  Tree Structured File Systems UNIX implements 2 services using trees UNIX implements 2 services using trees 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.  OS designers should provide DBMS facilities as lower level objects.

Scheduling, Process Management, and Interprocess Communication  Performance Task switches are inevitable Task switches are inevitable Task switches are expensive Task switches are expensive  Critical Sections Buffer pool is a shared data segment. Buffer pool is a shared data segment. De-Scheduling a lock-holding DB process causes problems. De-Scheduling a lock-holding DB process causes problems.  Server model Multi-Tasking Server Multi-Tasking Server First-Come-First-Serve Server First-Come-First-Serve Server

Consistency Control and Paged Virtual Memory  Consistency Control Buffer manager cannot be immune from knowledge of transactions. Buffer manager cannot be immune from knowledge of transactions. Buffer manager must maintain its own intentions list Buffer manager must maintain its own intentions list OS functions are duplicated. OS functions are duplicated.  Paged Virtual Memory Possibility of 2 page faults per I/O request Possibility of 2 page faults per I/O request Same problems the buffer pool management has. Same problems the buffer pool management has.

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.

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 Operating Systems have advanced considerably since then. His points may no longer be applicable.

Changes if Rewritten Today  Increase the diversity of OS 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.

Questions?