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Oracle Option: Real Application Clusters (RAC)

RAC is the computer cluster version of Oracle Database: Multiple instances share the same (shared) database. The instances are connected by an interconnect.
Reasons to use RAC are
RAC requires Oracle Clusterware

ADR

In a RAC environment, each participating instance and ASM have their own ADR home.

Checking if an instance is a RAC instance

Is background process LMON running: 
$ ps -ef | grep lmon | grep $ORACLE_SID
Does SQL*Plus show Real Application Clusters in its banner when started up?
Is the value of the init parameter cluster_database = true (note that during a maintenance period, the value may be false even though its a RAC instance).
The following shell command checks if the RAC option is linked. If it does not return anything, it is not linked, otherwise, it prints kcsm.o: 
$  ar -t $ORACLE_HOME/rdbms/lib/libknlopt.a | grep kcsm.o

See also

RAC One Node
Oracle database options
Dictionary views for RAC are created with catclust.sql
gv$ views.
There is also the object type cluster which must be distinguished from RAC.

Index

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