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Step 5 - Instance Memory Structure

Learn Oracle - Instance Memory Structure

This section briefly describes the memory structures of an instance. The size of these structures affects the performance of the Oracle database server and is controlled by initialization parameters. These initialization parameters can be categorized as memory parameters.

When a database is created with DBCA, the memory parameters are automatically set to optimal values based on your specification of the database workload. However, as your database usage expands, Oracle DBA might find it necessary to alter the settings of the memory parameters.

Oracle provides alerts and advisors to identify memory sizing problems and to help Oracle DBA determine appropriate values for memory parameters.

The System Global Area (SGA)

The SGA is a shared memory area that contains data and control information for the instance. Multiple users can share data within this memory area (controlled by Oracle) and information stored in the SGA can avoid repeated access from physical disk, a time consuming operation.

For optimal performance, the SGA should be large enough to avoid frequent disk reads and writes.

The SGA has several subcomponents as listed :

Component Description

Buffer Cache

Before any data stored in the database can be queried or modified, it must be read from disk and stored in memory. The buffer cache is the component of the SGA that acts as the buffer to store any data being queried or modified. All user processes connected to the database share access to the buffer cache.

Shared Pool

The shared pool caches information that can be shared among users.

Some examples:

SQL statements are cached so that they can be reused

Information from the data dictionary such as user account data, table and index descriptions, and privileges is cached for quick access and reusability

Stored procedures, which are executable code that is stored in the database, can be cached for faster access

Redo Log Buffer

This buffer improves performance by caching redo information (used for instance recovery) until it can be written at once and at a more opportune time to the physical redo log files that are stored on disk.

Large Pool

This is an optional area that is used for buffering large I/O requests for various server processes.

Java Pool

The Java pool memory is used for all session-specific Java code and data within the Java Virtual Machine (JVM)

Program Global Area (PGA)

A program global area (PGA) is a memory area used by a single Oracle server process. A server process is a process that services a client’s requests. Each server process has its own private PGA area that is a nonshared area of memory created by Oracle when a server process is started.

The PGA is used to process SQL statements and to hold logon and other session information.

The amount of PGA memory used and its content depends on the instance configuration, that is, whether the instance is running in dedicated server or shared server mode.
Oracle Tutorial - BLOGS

STEP 5 - Next Topics
This step discusses starting and stopping the database instance, and managing initialization parameters.Learn Oracle DBA Step 5 by clicking links below:
Instance Memory StructureConfiguring Network
Oracle Background Processes
Accessing the Database
Shuting down and Restarting the instance and Database
Viewing and Modifying Initialization
Managing Memory Parameters
Oracle Database - PODCASTS
More Tutorials on Oracle dba ...

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Source : Oracle Documentation | Oracle DBA

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