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 The Architecture of Oracle 11g RAC
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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.

The Architecture of RAC

In this chapter, the architecture of Oracle 11g RAC is examined in further detail. RAC is a multi-instance single database.  The database data files of an Oracle database usually have the .dbf   file extension.  In RAC, these .dbf data files reside on a shared disk.  The shared disk file system must be a cluster aware file system. 


One way to better visualize RAC architecture is to redefine what a computer or server is.  A typical server has the following components:

  • Cabinet /chassis

  • Processors/CPUs (Central Processing Units)

  • Local hard drives

  • Memory/RAM (Random Access Memory)

  • System bus

  • Public network adapters/Public Ethernet adapters

  • Public network link

Additional Server Components for RAC

A server that is part of an Oracle RAC environment has these additional components:

  • Shared hard drives

  • Cache fusion

  • Oracle Clusterware

  • Private network adapters / Private Ethernet adapters

  • Private network link (InterConnect)

These components will be explained briefly here with cache fusion and clusterware being covered in more detail later in this chapter.


Before shared disks and clustered file systems were a common part of a data center, a computer or server was defined on an individual or discrete basis.  Like other clustered systems, Oracle RAC redefines a server.  The opposite of individual and discrete is attached, combined, indistinct and joined.  The Google search platform is also a clustered system.  The hundreds of thousands of computers that make up the Google search platform combine/join together to create one massively powerful server.  In the same way, Oracle RAC combines multiple computers to present a single server view to the application and end user.


Cache fusion is a diskless cache coherency mechanism that provides copies of blocks directly from a holding instance’s memory cache, or local SGA buffer cache, to a requesting instance’s memory cache, or remote SGA buffer cache. A server’s local memory communicates with a local CPU via the system or memory bus.   In an Oracle RAC environment, the memory bus to CPU communication channel still exists, but the memory communication is extended across the others servers using cache fusion.   The memory or cache is fused together across the servers via Gigabit Ethernet, 10 Gigabit Ethernet or the InfiniBand protocol.  Gigabit Ethernet’s maximum speed is 125 MB/s.  10 Gigabit Ethernet and most InfiniBand implementations are about ten times faster than Gigabit Ethernet.


Oracle Clusterware allows clustering of servers providing the SSI (single system image).  Oracle Clusterware is the intelligence in Oracle RAC that ensures the required cooperation between cluster nodes. 


A single instance Oracle database can function properly with only one network adapter which is used by the public network.  A RAC database with its multiple instances hosted on multiple servers also requires a private network adapter on each server to communicate over the private RAC network. The private RAC network is referred to as the interconnect.  This private network link is the communication channel used by cache fusion.

Clusters and Grids Defined

Clusters and grids, which were briefly described in Chapter 1, have a different vision and different objectives. Clusters have static resources for a specific application. Grids, that can consist of multiple clusters and standalone servers, are dynamic resource pools and are shareable among many different applications and users. A grid does not assume that all servers in the grid are running the same set of applications.

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