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
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 |
Each instance has its own set |
Datafiles |
Accessed by only one instance |
Shared by all instances |
Control Files |
Accessed by only one instance |
Shared by all instances |
Online Redo Logfile |
Dedicated for write/read to |
Only one instance can write |
Archived Redo Logfile |
Dedicated to the instance |
Private to the instance but |
Flash Recovery Log |
Accessed by only one instance |
Shared by all instances |
Alert Log and Trace Files |
Dedicated to the instance |
Private to each instance, |
ORACLE_HOME |
Multiple instances on the same |
Same as single instance plus |
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.