Let’s talk about how to modify Oracle SGA

Oracle database is a very powerful relational database that can be used to store, process and manage large amounts of data. The performance of Oracle database depends on various factors, one of the important factors is the System Global Area (SGA). SGA is a shared memory area allocated to the entire Oracle database instance. It contains all shared memory structures required by the Oracle instance. This article will introduce the concept, function and how to modify Oracle SGA.

1. The concept and role of Oracle SGA
   SGA is the memory area used by Oracle database to store and share data cache. It is considered a core part of the Oracle database because the data cache stored in the SGA can greatly reduce the number of disk I/Os. Doing so can not only improve the performance of the Oracle database, but also reduce its resource usage and improve the stability of the system.

SGA contains all shared memory structures required by Oracle, such as database cache, shared pool, log cache, etc. Therefore, the size of SGA directly affects the performance of Oracle database. If the SGA is too small, the Oracle database will have to read data from the disk frequently, which will reduce the performance and efficiency of the database. On the contrary, if the SGA is too large, it will affect the stability of the system because it will occupy more memory resources.

2. How to modify Oracle SGA
   In the Oracle database, you can adjust the size of SGA by modifying the parameters of SGA. The size of Oracle SGA depends on the following parameters:

• DB_BLOCK_BUFFERS parameter: Specifies the total size of memory used to cache data blocks, in 8K units.
• SHARED_POOL_SIZE parameter: Specifies the total size of memory used to cache shared SQL and PL/SQL code.
• LARGE_POOL_SIZE parameter: Specifies the total size of memory used to cache large database blocks.
• JAVA_POOL_SIZE parameter: Specifies the total size of memory used to cache Java objects.
• PGA_AGGREGATE_TARGET parameter: Specifies the maximum size of the PGA (Process Global Area), which is used to store user process data and program data.

In modern Oracle database versions, you can view the current SGA size and the values ​​of these parameters by running the following command:

SELECT * FROM V$SGA;
SELECT * FROM V$PARAMETER WHERE NAME LIKE '%pool%';
SELECT * FROM V$PGASTAT;

Then, you can use the following two There are three ways to modify the size of SGA:

Method 1: Use the SGA_TARGET parameter
For Oracle 10g and above, you can use the SGA_TARGET parameter to specify the size of the SGA, which will specify the size of the SGA memory. And the proportion allocated to each memory structure in SGA. You can use the following statement to set the size of the SGA_TARGET parameter:

ALTER SYSTEM SET SGA_TARGET = <size>;

Where, is the size of the specified SGA_TARGET parameter. In Oracle 11g and above, it is recommended to use SGA_TARGET and MEMORY_TARGET parameters together to allocate according to the actual physical memory size. You can use the following statement to set the size of the MEMORY_TARGET parameter and SGA_TARGET parameter:

ALTER SYSTEM SET MEMORY_TARGET = <size> scope=spfile;
ALTER SYSTEM SET SGA_TARGET = <size> scope=spfile;

Method 2: Manually modify the value of each SGA parameter
You can manually modify the value of each SGA parameter to modify the SGA memory size the goal of. You can use the following statement to set the size of each SGA parameter:

ALTER SYSTEM SET DB_BLOCK_BUFFERS = <size> scope=spfile;
ALTER SYSTEM SET SHARED_POOL_SIZE = <size> scope=spfile;
ALTER SYSTEM SET LARGE_POOL_SIZE = <size> scope=spfile;
ALTER SYSTEM SET JAVA_POOL_SIZE = <size> scope=spfile;
ALTER SYSTEM SET PGA_AGGREGATE_TARGET = <size> scope=spfile;
ALTER SYSTEM SET SGA_MAX_SIZE = <size> scope=spfile;

Among them, specifies the size of each SGA parameter.

3. Problems you may face when modifying Oracle SGA
   In the process of modifying Oracle SGA, you may encounter some problems, such as:

• Memory Insufficient: If the SGA is too large, it may cause insufficient memory and cause system performance to decrease.
• System crash: If the SGA is too large, it may cause the system to crash and result in data loss.
• Performance issues: If the SGA is too small, it may cause the Oracle database to use disk I/O frequently, thus affecting its performance and efficiency.

Therefore, before modifying Oracle SGA, you should perform proper analysis and testing to determine the appropriate SGA size.

4. Summary
   SGA is one of the core parts of the Oracle database. It has an important impact on the performance and stability of the Oracle database instance. You can improve the performance and efficiency of Oracle database by modifying the size of SGA. Before modifying the SGA, you should perform proper analysis and testing to determine the appropriate SGA size and avoid problems you may encounter.

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