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RAC ARCHITECTURE



The Oracle Real Application Clusters (RAC) option was first released as part of Oracle 9.0.1 in the summer of 2001. It is considered by many within the Oracle community to be the most significant feature introduced in Oracle 9i.

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


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

Component
Single Instance Environment
RAC Environment

SGA

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
Datafiles
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 recovery
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.

ORACLE_HOME
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_HOME for all instances in a RAC environment.



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