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Cache Fusion

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.


Cache fusion is an important technology that transforms Oracle Real Application Cluster System (RAC) into a high-performance database cluster. The cache fusion method extends the scale out capabilities to applications that use Oracle RAC without requiring any additional investment in modifications to the existing applications.

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. It has all the bells and whistles of a single instance, which the DBA(s) understand very well. 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 9i 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 7.1.

REQUESTING INSTANCE

HOLDING INSTANCE

DIRTY BLOCK EXISTS IN HOLDING INSTANCE

CACHE COHERENCY METHOD

For Read

Read

Yes

Cache Fusion

For Read

Write

No

Soft Ping (read from disk)

For Read

Write

Yes

Cache Fusion

For Write

Write

Does Not matter

Ping (force disk write)

 

 

 

 

Table 7.1: 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, all these ping, soft ping, and false ping issues have been solved. With the RAC system release in 9i, cache fusion fully resolves write/write conflicts using the new architecture of resource coordination and global cache service.

 


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.

http://www.rampant-books.com/book_2004_1_10g_grid.htm


 

 
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