Oracle 11g RAC Overview

RAC Architecture

Oracle Real Application clusters allows multiple instances to access a single database, the instances will be running on multiple nodes. In a standard Oracle configuration a database can only be mounted by one instance but in a RAC environment many instances can access a single database.

Oracle’s RAC is heavy dependent on an efficient, high reliable high speed private network called interconnected nodes, make sure when designing a RAC system that you get the best that you can afford.

The table below describes the difference of a standard oracle database (single instance) an a RAC environment


Single Instance Environment

RAC Environment


Instance has its own SGA

Each instance has its own SGA

Background processes

Instance has its own set of
background processes

Each instance has its own set
of background processes


Accessed by only one instance

Shared by all instances
(shared storage)

Control Files

Accessed by only one instance

Shared by all instances
(shared storage)

Online Redo Logfile

Dedicated for write/read to
only one instance

Only one instance can write
but other instances can read during recovery and archiving. If an instance is
shutdown, log switches by other instances can force the idle instance redo logs
to be archived

Archived Redo Logfile

Dedicated to the instance

Private to the instance but
other instances will need access to all required archive logs during media

Flash Recovery Log

Accessed by only one instance

Shared by all instances
(shared storage)

Alert Log and Trace Files

Dedicated to the instance

Private to each instance,
other instances never read or write to those files.


Multiple instances on the same
server accessing different databases ca use the same executable files

Same as single instance plus
can be placed on shared file system allowing a common ORACLE_HOM E for all
instances in a RAC environment.

Cluster : A cluster is a group of two or more interconnected computers or servers that appear as if they are one server to end users and applications and generally share the same set of physical disks.

Benefit : Provides a highly available framework where the failure of one node (for example a database server running an instance of Oracle) does not bring down an entire application. In the case of failure with one of the servers, the other surviving server (or servers) can take over the workload from the failed server and the application continues to function normally as if nothing has happened.


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