How to set up oracle sga

Oracle SGA settings

Oracle SGA (System Global Area) is a system-level shared memory and one of the most important memory areas in an Oracle database instance. SGA contains the memory area common to all database instance processes (Process), including data buffers, redo log buffers, shared pools, Java pools, etc. The size of Oracle SGA has a huge impact on the performance and stability of the database, so the setting of SGA is an aspect that database administrators need to focus on and optimize.

Oracle database allows users to manage the size of SGA in two ways: manual setting and automatic management (automatic memory management).

Manually set the SGA size

Manually setting the SGA size requires accessing the Oracle database configuration file init.ora (in versions before Oracle 9i) or spfile (in Oracle 10g and later versions).

Manually setting the size of SGA requires consideration of the following aspects:

1. Calculate the size of SGA
   Before manually setting SGA, you first need to calculate the size to be allocated to SGA . This size needs to be set according to the actual situation, and can be considered based on the following aspects:

• Calculate the number of user sessions
• Calculate the memory objects required for the database
• Configuring SGA space

Among them, calculating the number of user sessions and calculating the memory objects required for the database require certain calculations and estimates, which will not be described here.

The space for configuring SGA can be calculated according to the following formula:

SGA = Database Buffer Cache Shared Pool Large Pool Redo Log Buffer Java Pool Streams Pool

Among them, Database Buffer Cache is a data buffer, used to buffer data blocks in the database; Shared Pool is a shared pool, used to store shared SQL and PL/SQL code; Large Pool is a large pool, used to store larger memory objects; Redo Log Buffer is a redo log cache area, used to store user operation records; Java Pool is a Java pool, used to store Java objects; Streams Pool is a Streams pool, used to store the memory required by STREAMS.

2. Modify initialization parameters
   The size setting of SGA requires modifying the initialization parameter file of the Oracle database. You can modify the $ORACLE_HOME/dbs/init.ora or spfile.ora file. to fulfill. Generally speaking, it is recommended to use spfile files to set initialization parameters.

Set the size of SGA in spfile through the following command:

ALTER SYSTEM SET sga_max_size = xxxM SCOPE=SPFILE;
ALTER SYSTEM SET sga_target = xxxM SCOPE= SPFILE;

Among them, sga_max_size represents the maximum value of SGA, and sga_target represents the expected value of SGA. After the settings are completed, you need to restart the database to take effect.

Automatically manage SGA size

Oracle database’s Automatic Memory Management (AMM) is a new feature after Oracle 11g, which can dynamically manage SGA and PGA (Process Global Area) the size of. AMM can automatically allocate and adjust the size of SGA and PGA according to the needs of the database, without manual settings. In AMM, any parameters that set SGA will be ignored.

To enable AMM, you need to perform the following steps:

1. Set the SGA_TARGET parameter
   Add the following parameter settings to the database to enable AMM:

ALTER SYSTEM SET sga_target=xxxM SCOPE=SPFILE;

where xxx is the SGA memory size in MB.

2. Restart the database instance
   SGA_TARGET modification needs to restart the database instance for it to take effect.

Summary

SGA is one of the most important memory areas in the Oracle database. It has a great impact on the performance and stability of the database and requires the attention of database administrators. Manually setting the SGA size requires calculating the actual requirements of the database and modifying the initialization parameter file of the Oracle database according to the requirements. Automatically manage SGA size can dynamically manage the size of SGA and PGA without manual settings.

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