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 Oracle Database Buffer Cache
Oracle Tips by Burleson Consulting

Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

Database Buffer Cache

The database buffer cache holds copies of the data blocks read from the data files.  Access to the database buffer cache is shared.

 

From Oracle version 8 onwards, the buffer cache contains three buffer pools for different types of data usage. They are DEFAULT, KEEP, and RECYCLE. These three buffer pools have separate allocations of buffers and LRU lists that manage buffers.

  • The RECYCLE buffer pool is used to store blocks that are virtually never used after the initial read

  • The KEEP pool is for blocks that are referenced frequently

  • The DEFAULT buffer pool contains objects that are not assigned to any buffer pool and objects that are explicitly assigned to the DEFAULT pool.  Direct insert and direct read operations used for data loading, sorting, or hashing operations bypass buffer pools.

In Oracle 11g RAC, the database block buffers from each instance, through cache fusion, are merged to form a massive logical database buffer cache.

Large Pool

The large pool is an optional memory area in the SGA.  This separate memory area is beneficial for memory intensive tasks such as backup and restore operations. 

JAVA Pool

The JAVA Pool holds the JAVA execution code in a similar manner to the PL/SQL cache in the shared pool. The JAVA pool is used by many internal routines.

Streams Pool

The streams pool is used exclusively by Oracle Streams.  To configure the streams pool explicitly, specify the size of the pool in bytes using the streams_pool_size initialization parameter.  If the size of the streams pool is zero or not specified, then the memory used by streams is allocated from the shared pool.

Redo Log Buffers

Redo log buffers are used to hold the redo records generated by each data changing transaction. The redo log buffer is a circular buffer. Redo entries contain the steps needed to reconstruct changes made to the database by UPDATE, INSERT, CREATE, DELETE, ALTER, and DROP operations. A redo log buffer is written out to the online redo log by the log writer process when:

  • The buffer becomes one third full

  • Three seconds have elapsed

  • When a DBWn process writes modified buffers to disk

  • On commit record - when a user process commits a transaction

While it is possible to have different sized redo log buffers and redo logs on each instance in a RAC database, this is not a suggested configuration. It can lead to confusion and misunderstanding during recovery operations. Each instance in a RAC database must have its own thread of redo logs.

Shared Pool

The shared pool holds the dictionary cache, library cache, result cache, message queues, latch and lock areas, buffers for parallel execution messages, and control structures.

 

The data dictionary is a collection of internal tables and views of reference information about the database, the structure and users.

The dictionary cache is the memory area designated to hold dictionary data. It is also known as the row cache because it holds data as rows instead of buffers that hold entire blocks of data.

 

The library cache includes the shared SQL areas, PL/SQL procedures and packages, and control structures such as locks and library cache handles.

Fixed SGA

A portion of the SGA contains general information about the state of the database and the instance, which the background processes need to access. This is called the fixed SGA. No user data is stored here. The SGA also includes information communicated between processes, such as locking information.

     

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