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Oracle MGA Explained: The Flexible Memory Area
If you've managed an Oracle database, you know the classic memory duo: the SGA (System Global Area) for shared data and the PGA (Program Global Area) for private session work. They've been the foundation of Oracle's performance for decades.
But what if there's a third player? A more agile, dynamic memory area designed for the modern, feature-rich Oracle database?
Meet the Managed Global Area (MGA), a powerful memory framework introduced in Oracle 18c and enhanced in later versions like 23c (23ai). If you're running 18c or above, the MGA is already working in your system, and understanding it is crucial for optimal performance and configuration—especially when planning an upgrade.
In this post, we will clarify the MGA. You'll discover what it is, how it connects the SGA and PGA, and the one key configuration change you need to make to prevent memory errors.
SGA and PGA: A Quick Refresher
Before we explore the MGA, let's quickly revisit the classics. Think of them as the established roles in a well-organized kitchen.
SGA (System Global Area): This is the shared kitchen workspace. It houses frequently used data (Buffer Cache), execution plans (Shared Pool), and other instance-wide control structures. All processes share this space. It's created once when the instance starts and remains the same size.
PGA (Program Global Area): This serves as each chef's private cutting board. This memory is unique to each server process, used for tasks like sorting, hashing, and storing session-specific information. It can change in size but remains isolated.
For a long time, this system worked well. But what happens when you need a semi-shared space? A temporary, flexible area where a certain group of chefs can collaborate on a complex dish without overcrowding the main counter? This is the issue the MGA addresses.
What is the Managed Global Area (MGA)?
The Managed Global Area (MGA) is a flexible memory area designed for a specific set of trusted Oracle processes. It serves as a framework for new and future database features that don't fit neatly into the rigid SGA/PGA model.
Think of the MGA as a set of temporary worktables that can be quickly set up and taken down in the kitchen.
A team of chefs (processes) can pull up a table (a namespace) to work together on a complex recipe (e.g., a parallel query).
They can share ingredients and results on this table effectively.
Once the dish is complete, the table is cleared away, and the space is reclaimed.
This "on-demand" quality is what makes the MGA so effective.
Key Characteristics of the MGA:
- Elastic: Memory segments are created and removed as needed.
- Shared but Selective: It can be shared by all processes or just a specific group working on a task.
- Structured as Namespaces: The MGA is divided into namespaces, each dedicated to a specific feature or component (e.g., Vector Cache, Multitenant metadata).
- Counted under PGA: This is crucial! The total memory used by the MGA counts against your PGA_AGGREGATE_LIMIT.
MGA vs. SGA vs. PGA: A Clear-Cut Comparison
The table below highlights the key differences at a glance.
Why Should You Care? The Practical Impact of MGA
The Upgrade Pitfall: Hitting the PGA Wall
Here's the most important takeaway for any DBA: When you upgrade from a version before 18c to 18c or later, your existing PGA_AGGREGATE_LIMIT may now be too low.
Why? Because the MGA's memory use is deducted from the total budget for the PGA. If you don't consider this, your database could hit the PGA limit and start terminating sessions, even if your workload hasn’t changed.
You can check the MGA's memory footprint with a simple query:
SQL> select name, value from v$pgastat where name like 'MGA%';
NAME VALUE
---------------------- ----------------
MGA allocated (under PGA) 6442450944 -- That's 6 GB of your PGA limit!
New PGA_AGGREGATE_LIMIT = (Old PGA_AGGREGATE_LIMIT) + (Max # of Processes * 5 MB)
You can find the maximum number of processes your instance has used with:
select max_utilization from v$resource_limit where resource_name='processes';
Method 2: The Scientific Approach
For a more precise calculation based on your instance's actual usage, you can use this provided query. It calculates a new recommended limit by considering your historical maximum PGA usage and process count.
WITH
MAX_PGA as
(select round(value/1024/1024,1) max_pga from v$pgastat where name='maximum PGA allocated'),
MGA_CURR as
(select round(value/1024/1024,1) mga_curr from v$pgastat where name='MGA allocated (under PGA)'),
MAX_UTIL as
(select max_utilization as max_util from v$resource_limit where resource_name='processes')
SELECT
a.max_pga "Max PGA (MB)",
b.mga_curr "Current MGA (MB)",
c.max_util "Max # of processes",
round(((a.max_pga - b.mga_curr) + (c.max_util * 5)) * 1.1, 1) "New PGA_AGGREGATE_LIMIT (MB)"
FROM MAX_PGA a, MGA_CURR b, MAX_UTIL c;
Real-World Use Cases: What is the MGA For?
The MGA isn't an abstract concept; it's already powering specific Oracle features.
Parallel Queries: The Query Coordinator (QC) can share an MGA namespace with its parallel execution servers to efficiently share intermediate query results, making complex data processing faster.
Vector Cache: In Oracle 23ai, the MGA is used to store vector data in memory, which is essential for the blazing-fast performance of Vector Search and AI-based queries.
Future-Proofing: The MGA's namespace architecture is designed for extensibility. It allows Oracle to roll out new in-memory features without having to redesign the core SGA/PGA, meaning we'll see more of it in future releases.
Key Takeaways and Next Steps
The Managed Global Area is a significant evolution in Oracle's memory architecture.:
MGA is a Third Memory Area: It's a flexible, elastic space that sits conceptually between the SGA and PGA.
It's for Selective Sharing: It allows a defined set of processes to share memory dynamically.
Check Your PGA Limit: If you've upgraded to 18c or above, you must check and likely increase your PGA_AGGREGATE_LIMIT to account for MGA usage.
It's the Foundation for the Future: Features like the Vector Cache rely on the MGA, making it a cornerstone of modern Oracle performance.
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