Changing SGA in Oracle Database Instance
- Vinay Shree Shukla
- Feb 12
- 4 min read
Updated: Apr 2
Changing the System Global Area (SGA) in an Oracle database is a critical task for database administrators (DBAs) to optimize database performance. The SGA plays a key role in storing data and control information for an Oracle database instance. Managing its size properly ensures efficient memory usage, improves query execution time, and prevents performance bottlenecks.
This guide provides a step-by-step approach to adjusting the SGA size in an Oracle Database instance, covering prerequisites, memory parameter details, dynamic modifications, and essential best practices.
What is SGA?
The System Global Area (SGA) is a shared memory region allocated by an Oracle database instance at startup. It consists of several components, each serving a specific purpose in database operations. The key components of SGA include:
Database Buffer Cache: Caches frequently accessed data blocks read from disk, reducing disk I/O.
Shared Pool: Stores SQL execution plans, PL/SQL code, data dictionary information, and other critical metadata.
Redo Log Buffer: Holds redo entries temporarily before writing them to redo log files for recovery purposes.
Large Pool, Java Pool, and Streams Pool: Optional memory areas used for specific database functions such as shared server processes, Java execution, and Oracle Streams.
Effective SGA management directly impacts database performance, making it essential for DBAs to adjust it as needed.
Prerequisites for Changing SGA
Before modifying SGA settings, ensure the following:
Database Instance: The Oracle instance you want to modify must be running.
Privileges: You need SYSDBA privileges to alter memory settings.
Automatic Memory Management (AMM) Settings: Determine whether Automatic Memory Management (AMM) or Automatic Shared Memory Management (ASMM) is enabled. AMM and ASMM influence how SGA components are allocated.
Physical Memory Availability: Ensure the new SGA size does not exceed available physical memory to prevent system performance issues.
Steps to Modify SGA in Oracle Database
1. Checking Current SGA Settings
Before making any modifications, check the existing SGA settings using the following SQL command:
SHOW PARAMETER sga;
For more detailed information about SGA components, use:
SELECT component, current_size, min_size, max_size
FROM v$sga_dynamic_components;
This query provides insights into the current allocation of SGA components and helps determine necessary adjustments.
2. Understanding Key Memory Parameters
Oracle uses several parameters to control memory allocation within the SGA:
SGA_TARGET: Specifies the total size allocated for SGA.
SGA_MAX_SIZE: Defines the maximum allowable size of SGA across instance startups.
DB_CACHE_SIZE: Determines the size of the Database Buffer Cache.
SHARED_POOL_SIZE: Defines the memory allocated for the Shared Pool.
REDO_LOG_BUFFER: Specifies the size of the Redo Log Buffer.
If AMM is enabled, two additional parameters control memory allocation:
MEMORY_TARGET: Specifies the total amount of memory allocated to Oracle (including both SGA and PGA).
MEMORY_MAX_TARGET: Sets the upper limit for MEMORY_TARGET.
3. Modifying SGA Settings Dynamically
If SGA_TARGET is less than or equal to SGA_MAX_SIZE, you can adjust it dynamically without restarting the database.
For example, to increase SGA_TARGET by 1GB:
ALTER SYSTEM SET SGA_TARGET = 1G SCOPE=BOTH;
SCOPE=BOTH applies the change immediately and makes it persistent across restarts.
SCOPE=SPFILE saves the change for future restarts but does not apply it immediately.
4. Adjusting SGA_MAX_SIZE (Requires Restart)
If you need to increase SGA_MAX_SIZE, a database restart is required. Modify the value as follows:
ALTER SYSTEM SET SGA_MAX_SIZE = 3000M SCOPE=SPFILE;
After making this change, restart the database for it to take effect:
SHUTDOWN IMMEDIATE;
STARTUP;
SHOW PARAMETER SGA;
5. Modifying Individual SGA Components
If SGA_TARGET is not set, or if you need to fine-tune specific components within its limits, modify them individually:
Increasing Database Buffer Cache:
ALTER SYSTEM SET DB_CACHE_SIZE = 200M SCOPE=BOTH;
Adjusting Shared Pool Size:
ALTER SYSTEM SET SHARED_POOL_SIZE = 200M SCOPE=BOTH;
These adjustments ensure proper memory allocation for frequently used queries and metadata caching.
6. Verifying Changes
After making modifications, verify the new SGA settings by running:
SHOW PARAMETER sga;
To check the status of dynamic memory components:
SELECT component, current_size FROM v$sga_dynamic_components;
Monitoring these settings ensures that your changes are correctly applied and that memory is optimally allocated.
Important Considerations
When modifying SGA settings, keep the following best practices in mind:
1. Avoid Over-allocating Memory
Allocating excessive memory to SGA beyond available physical RAM can lead to swapping, which severely impacts performance. Always ensure the new memory settings fit within the system’s available memory.
2. Understand SGA_MAX_SIZE Limitations
Once SGA_MAX_SIZE is set, it cannot be decreased dynamically. Any increase in SGA_MAX_SIZE requires a database restart.
3. Test Changes in a Development Environment
Before applying changes in production, test them in a staging or development environment. This prevents unexpected performance degradation and helps optimize memory allocation without affecting live operations.
4. Use Automatic Memory Management (AMM) if Suitable
If managing SGA manually is complex, consider enabling Automatic Memory Management (AMM) by setting MEMORY_TARGET. This allows Oracle to dynamically adjust memory between SGA and PGA as needed.
Conclusion
Tuning the System Global Area (SGA) is a crucial part of Oracle database performance optimization. By understanding SGA components, adjusting memory settings dynamically, and following best practices, database administrators can significantly enhance database performance.
SGA_TARGET enables flexible memory allocation without restarting the database.
SGA_MAX_SIZE defines the upper limit but requires a restart for changes.
Fine-tuning individual components like DB_CACHE_SIZE and SHARED_POOL_SIZE ensures efficient memory utilization.
Always verify settings and test changes before implementing them in production.
By leveraging Automatic Shared Memory Management (ASMM) or Automatic Memory Management (AMM), you can simplify memory tuning while ensuring optimal database performance. Regular monitoring and adjustments will help maintain a well-optimized Oracle database system.
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