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  Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 7 - Cache Fusion and Inter Instance Coordination

Database Buffer Cache

This area of the SGA contains only the buffers themselves and not their control structures. For each buffer, there is a corresponding buffer header in the variable area of the SGA.

Program Global Area (PGA)

A program global area (PGA) is a memory region that contains data and control information for a server process. It is a non-shared memory created by Oracle when a server process is started. Access to the PGA is exclusive to that server process and it is read and written only by Oracle code acting on its behalf. It contains a private SQL area and a session memory area.

A private SQL area contains data such as bind information and runtime memory structures. Each session that issues a SQL statement has a private SQL area. Session memory is the memory allocated to hold a session's variables (logon information) and other information related to the session. For a shared server, the session memory is shared and not private.

Buffer Cache Management

The database buffer cache is organized in two lists: the write list and the least-recently-used (LRU) list. The write list holds dirty buffers, which contain data that has been modified but has not yet been written to disk. The LRU list holds free buffers, pinned buffers, and dirty buffers that have not yet been moved to the write list. Free buffers do not contain any useful data and are available for use. Pinned buffers are buffers that are currently being accessed.

The above text is an excerpt from:

Oracle 10g Grid & Real Application Clusters
Oracle 10g Grid Computing with RAC
 ISBN 0-9744355-4-6

by Mike Ault, Madhu Tumma

 

   
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