Oracle SGA setup methods and precautions

Oracle SGA (System Global Area) is the most important system memory area in the Oracle database. It contains all the memory structures required when the database instance is running, such as data cache, shared pool, redo log buffer, etc. Correctly setting up SGA has a great impact on database performance and can significantly improve query and transaction processing speeds. This article will introduce the setting methods and precautions for Oracle SGA.

1. SGA components

In Oracle SGA, it mainly includes the following components:

(1) Buffer cache: used to cache the latest The data blocks used reduce the frequency of disk access and increase the speed of data access.

(2) Shared pool: stores shared SQL and PL/SQL areas to improve repeated operation performance.

(3) Log buffer: used to temporarily store the transaction log before submission to ensure the consistency and persistence of the transaction.

(4) Large pool: Mainly used to store large memory blocks required by I/O server processes and other processes, such as sorting and hash operations.

2. Set SGA

Setting the SGA size is based on the system’s hardware resources and application requirements. There are two ways to manually set the SGA: dynamically adjusting the SGA size and statically setting the SGA size.

Dynamic adjustment of SGA size has been enabled by default after Oracle 10g. If you need to run it manually, you can use the following SQL command:

ALTER SYSTEM SET SGA_TARGET = target_size SCOPE = MEMORY;

Among them, target_size is the parameter that specifies the SGA size, and SCOPE is used for Specify the setting level, MEMORY means it is only valid for the current instance. The SGA_TARGET parameter can be adjusted dynamically, that is, the SGA size is modified while the instance is running, and Oracle will automatically allocate memory dynamically according to the needs of the application. The SGA_SIZE parameter can also be used to set the SGA size, but it needs to be restarted to take effect. It is not recommended.

If you want to disable dynamic adjustment of the SGA size, you can specify the SGA_TARGET parameter as 0, and then use the following SQL command to permanently set the SGA size to a static value:

ALTER SYSTEM SET SGA_MAX_SIZE = max_size SCOPE = SPFILE;

Among them, max_size specifies the SGA The maximum size of the SPFILE representation is set in the parameter file.

It should be noted that the size of SGA cannot exceed the available memory size of the operating system where the instance is located, otherwise it will cause the instance to crash or the database to slow down. Generally speaking, when the system is not busy, it is recommended to set the SGA size to 50% to 70% of the operating system's available memory.

3. SGA size optimization

Properly setting the SGA size is critical to database performance, but it also needs to be adjusted and optimized according to specific circumstances. The following are some suggestions for optimizing SGA size:

(1) Monitor SGA usage: Use the tools provided by Oracle (such as v$sga_dynamic_components and v$sga_resize_ops views) to monitor SGA usage so that the size can be adjusted in a timely manner .

(2) Adjust automatic PGA management parameters: Automatic PGA management can be controlled using the PGA_AGGREGATE_TARGET parameter. If this parameter is set, Oracle will automatically manage the program global area (PGA) and optimize memory usage.

(3) Reasonable allocation of memory: buffer cache, shared pool and large pool are only some components of SGA. Depending on the needs of your application, you can increase the size of a component to optimize database performance.

(4) Consider using NUMA architecture: If the server uses NUMA architecture, consider the memory access method of SGA to make full use of hardware resources.

4. Summary

The setting of SGA is one of the important factors in Oracle database performance optimization. Properly setting SGA can improve the performance of the database and shorten the time of query and transaction processing. . Using dynamic SGA adjustment allows Oracle to automatically adjust memory and optimize memory usage. However, it needs to be adjusted according to the application usage requirements on the premise of rational use of hardware resources. By monitoring SGA usage, the SGA size can be adjusted in real time to ensure the normal operation of the database.

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