Modify oracle sga

Oracle's SGA (System Global Area) is a memory area shared by all processes. It stores a large number of important data structures such as caches, locks, connections, etc. The size of SGA directly affects the performance of Oracle database. If the SGA is set too small, it will not be able to accommodate the data required by all processes, resulting in frequent disk reads and writes and a large number of I/O operations, which will affect the response time of queries and transactions. If it is set too large, it will occupy too much memory resources and cause the operating system to crash.

Therefore, for an Oracle database in a production environment, reasonable SGA settings are crucial. This article will explore how to modify Oracle SGA.

1. Confirm the current size of SGA

Use the following query statement in the SQL*Plus command line interface:

SHOW PARAMETER SGA_TARGET;

You can view the size of the current SGA_TARGET parameter . This value specifies Oracle's SGA target size, in bytes. Note that this value may differ from the actual memory size allocated to the SGA. Therefore, we also need to use the following query statement to get the actual SGA size used:

SELECT * FROM V$SGA;

After executing this statement, the current SGA situation will be output, where the "Size" column displays the total size of SGA.

2. Calculate the required SGA size

Before modifying the SGA, we need to calculate the required SGA size first. This process needs to consider many factors, such as database size, number of connections, cache hit rate, concurrency, etc. Here, we use a simple formula to calculate a preliminary SGA size:

SGA = (DB_BLOCK_SIZE * DB_BLOCK_BUFFERS) + SHARED_POOL_SIZE + LARGE_POOL_SIZE + JAVA_POOL_SIZE + PGA_AGGREGATE_TARGET

Among them, DB_BLOCK_SIZE is the size of each data block, DB_BLOCK_BUFFERS is the number of blocks in the data block cache area, and SHARED_POOL_SIZE is the shared pool Size, LARGE_POOL_SIZE is the size of the large pool and variable pool, JAVA_POOL_SIZE is the size of the Java pool, and PGA_AGGREGATE_TARGET is the PGA target size. The values ​​of these parameters can be obtained by querying the V$PARAMETER table. It should be noted that the value of DB_BLOCK_BUFFERS should be an n-th power of 2, and its value should satisfy: SGA PGA other memory <= available memory * 90%.

For example, if we require SGA to be 400MB, DB_BLOCK_SIZE to be 8KB, DB_BLOCK_BUFFERS to be 50176, SHARED_POOL_SIZE to be 60MB, LARGE_POOL_SIZE to be 10MB, JAVA_POOL_SIZE to be 30MB, and PGA_AGGREGATE_TARGET to be 50MB, then we can get:

SGA = (8KB * 50176) + 60MB + 10MB + 30MB + 50MB
    = 400MB

Therefore, we can set the value of the SGA_TARGET parameter to 400MB to meet our needs.

3. Modify the size of SGA

To modify the size of SGA, we need to edit Oracle's initialization parameter file (such as pfile or spfile). Here, we take modifying spfile as an example to illustrate. Please note that modifying the initialization parameter file can have a significant impact on the database and must be done with caution. Before making modifications, please back up the initialization parameter file and database.

First, connect to the Oracle database through SQL*Plus. Then use the following command to check whether there is an spfile:

SHOW PARAMETER SPFILE;

If the output shows that the spfile already exists, then we can modify the spfile directly. If the output result is empty, then we need to use the following command to create the spfile:

CREATE SPFILE FROM PFILE;

Next, we can use the following command to modify the size of SGA:

ALTER SYSTEM SET SGA_TARGET = 400M SCOPE=SPFILE;

This command will change the SGA_TARGET parameter Change the value to 400MB and write the changes to the spfile. At this point, we need to use the following command to restart the database for the modification to take effect:

SHUTDOWN IMMEDIATE;
STARTUP;

Through the above steps, we have successfully modified the size of Oracle SGA.

Summary

Oracle SGA is one of the keys to database performance, so its reasonable setting is crucial. This article introduces the steps to modify Oracle SGA. I hope it will be helpful to everyone. Of course, before modifying SGA, we should also understand other important concepts and parameters, such as PGA, SGA_MAX_SIZE, SGA_TARGET, MEMORY_MAX_TARGET, etc., in order to make more reasonable SGA settings.

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