What are the key components of the Oracle System Global Area (SGA) and their respective functions?

Oracle SGA is composed of multiple key components, each of which undertakes different functions: 1. Database Buffer Cache is responsible for caching data blocks to reduce disk I/O and improve query efficiency; 2. Redo Log Buffer records database changes to ensure transaction persistence and recovery capabilities; 3. Shared Pool includes Library Cache and Data Dictionary Cache, which is used to cache SQL parsing results and metadata; 4. Large Pool provides additional memory support for RMAN, parallel execution and other tasks; 5. Java Pool stores Java class definitions and session objects; 6. Streams Pool is used for data replication and message queues of Oracle Streams. Properly configuring these components can significantly improve database performance.

Oracle's System Global Area (SGA) is one of the most important memory structures in Oracle instances, and it is a memory area shared by all database processes. Understanding the key components of SGA and its functions is very helpful in tuning database performance and troubleshooting issues.

Let’s take a look at what parts SGA mainly consist of and what role they play.

1. Database Buffer Cache: The core of data cache

The function of this area is to cache data blocks read from the data file, which can reduce disk I/O and improve query efficiency.

• When a user performs SELECT or other DML operations, Oracle first searches for the required data blocks in the Buffer Cache.
• If it cannot be found (that is, cache miss), it will be read from disk and put into the cache.
• Data modification will also be performed in the Buffer Cache before it is written to the data file (through the DBWn process).

Tips: Buffer Cache is too small and can cause frequent disk access, and too large may waste memory resources. You can judge whether you need to resize based on the "buffer hit rate".

2. Redo Log Buffer: Log cache for logging changes

Redo Log Buffer is a relatively small but very active memory area for temporarily storing redo information for database changes.

• All modification operations to database objects will generate Redo records, such as insertion, update, and deletion.
• These records are periodically written to the online Redo log file by the LGWR process.
• Its main purpose is to ensure the persistence of transactions and to support database recovery.

Note: If the system has a large number of concurrent write operations, the Redo Log Buffer may become a bottleneck. Appropriately increasing its size (via the LOG_BUFFER parameter) can help alleviate this problem.

3. Shared Pool: Shared space for SQL and PL/SQL

Shared Pool is one of the most complex parts of SGA, mainly consisting of two important components:

• Library Cache : Caches the execution plan and parsing results of SQL and PL/SQL statements.
• Data Dictionary Cache (also called Row Cache) : caches database metadata, such as table definitions, permissions and other information.

These two subcomponents work together to improve SQL parsing efficiency and avoid the overhead caused by repeated parsing.

suggestion:

• If you find that SQL is frequently reparseed (hard parse), it may be that the Shared Pool is not large enough.
• Using binding variables can effectively reduce the pressure on Shared Pool.

4. Large Pool (optional): auxiliary memory for handling large tasks

Large Pool is not necessary, but it is recommended to configure it if some of the following features are used:

• Backup and Recovery Operations (RMAN)
• Perform message buffering in parallel
• Session memory in shared server mode

It exists to avoid taking up resources from Shared Pool, thereby maintaining the stability of Shared Pool.

5. Java Pool: Serving Java Objects

If you run Java stored procedures or Java classes in your database, this part of the memory will be used.

• Used to store class definitions, session objects, etc. in Java virtual machine (JVM).
• Although Java is not commonly used in most modern databases, it is still important in a specific environment.

6. Streams Pool (optional): Stream replication related memory

When using Oracle Streams technology to implement data replication or message queueing, this part of the memory is used to cache related data change information.

• It usually needs to be enabled manually (set the STREAMS_POOL_SIZE parameter).
• If the Streams feature is not used, this part can be ignored.

Basically these are the main components. Each section has its own specific purpose, and configuring them reasonably can significantly improve database performance. Although many of these parameters can be managed automatically (for example in AMM or ASMM mode), understanding the working mechanism behind them is still essential for tuning and troubleshooting.

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