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 Oracle RAC Cache Fusion
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Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

Cache Fusion

The topics that will be covered in this section include the nature, internals, and working mechanism of cache fusion technology along with the following subjects:

  • Virtualization of multiple caches into a single cache

  • How the data blocks are moved across among multiple SGAs in a multi-node environment

  • Synchronization of resource access

  • Resource coordination methodology

  • Re-mastering of resources in the event of unforeseen failure of any instance

Cache fusion is a diskless cache coherency mechanism that provides copies of blocks directly from a holding instance’s memory cache (local SGA buffer cache) to a requesting instance’s memory cache (remote SGA buffer cache).


A RAC system equipped with low-latency and high speed interconnect technology enables the buffer cache of each node in the cluster to fuse and form into a single virtual global cache, hence the term cache fusion. The cache fusion architecture creates a shared-cache and provides a single cache image or view to the applications. Internals are transparent to the applications.


From a functional viewpoint, an instance in a RAC system is equivalent to a single instance of Oracle.  The extension of multiple cache buffers into a single fused global cache improves scalability, reliability, and availability.


While cache fusion provides Oracle users with an expanded database cache for queries and updates of I/O operations, the improved performance depends greatly on the efficiency of the inter-node message passing mechanism that handles the data block transfers.

Evolution of Cache Fusion

Before looking deeper into the implementation of cache fusion in Oracle 11g RAC, some time needs to be taken to look at the implementation in the 8i release. Oracle Release 8i (Oracle Parallel Server) introduced the initial phase of cache fusion. The data blocks were transferred from the SGA of one instance to the SGA of another instance without the need to write the blocks to disk. This was aimed at reducing the ping overhead of data blocks. However, the partial implementation of cache fusion in 8i could help only in certain conditions, as indicated in Table 2.1.






For Read



Cache Fusion

For Read



Soft Ping

(read from disk)

For Read



Cache Fusion

For Write


Does Not matter

Ping (force disk write)

Table 2.1: The Methods of Maintaining Cache Coherency

Oracle 8i (Oracle Parallel Server) had a background process called the Block Server Process (BSP), which facilitated cache fusion. BSP was responsible for transferring the required blocks directly from the owning instance to the buffer cache of the requested instance.


For read/write operations, if the block was already written to disk by the holding instance, the requested block was read from the disk. It involved a soft ping or an I/O-less ping. If the block was available on the holding instance buffer, the BSP process prepared a consistent read (CR) image of the data block. It was then sent to the requesting instance.


A write/write operation invariably involved the ping of the data block. When the ping occurred, the holding instance wrote to disk and downgraded the lock mode. Then, the requesting instance acquired the necessary lock mode and read from the disk. This frequent pinging hurt the performance of the OPS database. With the full implementation of cache fusion in release 9i, 10g, and 11g, all these ping, soft ping, and false ping issues have been solved. Cache fusion now fully resolves write/write conflicts using the new architecture of resource coordination and global cache service.

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