Cache Fusion is a critical feature of Oracle Real Application Clusters (RAC) that allows multiple instances to share data in a synchronized manner across a clustered database system. Here’s a detailed breakdown:

1. Data Sharing: In a RAC environment, multiple Oracle instances run on separate nodes but access the same shared database. Cache Fusion enables these instances to share data blocks directly from their cache (memory) rather than having to read them from disk.
2. Consistency: Cache Fusion ensures data consistency across all nodes. When a data block is updated by one instance, Cache Fusion transfers the modified block to other instances that need it, ensuring they all have the most up-to-date version.
3. Performance: By allowing instances to communicate directly and share data blocks in memory, Cache Fusion significantly reduces the need for disk I/O operations. This results in lower latency and improved performance for database operations.
4. Interconnect: The communication between instances is facilitated by the high-speed interconnect network in the cluster. This network enables rapid transfer of data blocks and synchronization messages.
5. Mechanism:

• Block Transfers: When an instance needs a data block that another instance has modified, Cache Fusion transfers the block through the interconnect.
• Lock Management: Oracle RAC uses a global lock manager to manage access to data blocks and ensure that only one instance can modify a block at any given time. When another instance needs to modify the same block, Cache Fusion coordinates the transfer and the lock change.
• Global Cache Service (GCS) and Global Enqueue Service (GES): These Oracle services manage the cache coherence and lock management across the cluster.

1. Scalability: Cache Fusion helps Oracle RAC to scale horizontally. As more nodes are added to the cluster, the system can handle increased workloads without a linear increase in disk I/O operations, maintaining efficiency and performance.

In summary, Cache Fusion is a key feature that allows Oracle RAC to function efficiently by enabling direct memory-to-memory data sharing and ensuring data consistency across all nodes in the cluster.

Let’s understand by example

Let’s walk through an example to illustrate how Cache Fusion works in an Oracle RAC environment.

Scenario

Imagine a two-node Oracle RAC setup with Node A and Node B. Both nodes are running Oracle instances accessing the same shared database.

Step-by-Step Example

1. Initial State:

• Node A and Node B both start with no cached data blocks related to a particular table, say EMPLOYEES.

1. First Access:

• A user connected to Node A runs a query to fetch data from the EMPLOYEES table.
• Node A reads the necessary data blocks from the shared disk and caches them in its memory.
• The fetched data blocks are now present in Node A’s cache.

1. Subsequent Access by Another Node:

• Shortly after, a user connected to Node B executes a similar query on the EMPLOYEES table.
• Instead of Node B reading the same blocks from the disk, Cache Fusion checks if Node A has the required data blocks in its cache.
• If Node A has the latest version of the data blocks, Node B requests these blocks from Node A.
• Node A sends the cached data blocks over the high-speed interconnect to Node B.
• Node B receives the blocks and stores them in its cache, avoiding disk I/O.

1. Data Modification:

• Now, suppose the user on Node B wants to update an employee’s salary in the EMPLOYEES table.
• Node B modifies the data block in its cache.
• Cache Fusion ensures that this modified block is sent to Node A, so Node A has the most up-to-date version of the block.
• This block transfer includes necessary locking mechanisms to maintain data consistency.

1. Consistency Management:

• If another user on Node A tries to update the same data before receiving the updated block from Node B, Cache Fusion ensures that Node A requests the latest block from Node B.
• Node B transfers the updated block to Node A.
• Node A receives the block, applies the modification, and both nodes now have consistent data.

Example in SQL Terms

1. Initial Query on Node A:

-- Connected to Node A

SELECT * FROM EMPLOYEES WHERE EMPLOYEE_ID = 100;

2. Subsequent Query on Node B:

-- Connected to Node B

SELECT * FROM EMPLOYEES WHERE EMPLOYEE_ID = 100;

3. Update on Node B:

-- Connected to Node B

UPDATE EMPLOYEES SET SALARY = 6000 WHERE EMPLOYEE_ID = 100; COMMIT;

4. Update on Node A:

-- Connected to Node A

UPDATE EMPLOYEES SET SALARY = 6500 WHERE EMPLOYEE_ID = 100; COMMIT;

Key Points

• Reduced Disk I/O: By transferring data blocks directly between nodes’ caches, Oracle RAC reduces the need for disk reads.
• Improved Performance: Memory-to-memory transfers over a high-speed interconnect are much faster than disk I/O operations.
• Data Consistency: Cache Fusion ensures that all nodes have the most current data, maintaining database integrity and consistency.

In summary, Cache Fusion enables efficient, consistent, and high-performance data sharing in an Oracle RAC environment, allowing multiple nodes to work on the same data seamlessly.