RAC Basics : CRS, Voting Disk, OCR, Cache Fusion ….

Oracle Clusterware (Cluster Ready Services in 10g/ Cluster Manager in 9i) – provides infrastructure that binds multiple nodes that then operate as single server. Clusterware monitors all components like instances and listeners. There are two important components in Oracle clusterware, Voting Disk and OCR (Oracle Cluster Registry).


Voting Disk – is file that resides on shared storage and Manages cluster members.  Voting disk reassigns cluster ownership between the nodes in case of failure.

OCR (Oracle Cluster Registry) – resides on shared storage and maintains information about cluster configuration and information about cluster database. OCR contains information like which database instances run on which nodes and which services runs on which database.

CRS Resource – anything that Oracle Clusterware manages is classified as CRS resource like database, instance, service, listener, VIP address and so on.

Cluster-Aware Storage – is storage solution for Oracle RAC like RAW device, OCFS, ASM… to know more about storage option in RAC click here
Interconnect – is private network that connects all the servers in cluster. Interconnect uses switch that only nodes in cluster can access. Instances in cluster communicate to each other via interconnect.

Cache Fusion – is disk less cache coherency mechanism in Oracle RAC that provides copies of data blocks directly from one instance’s memory cache (in which that block is available) to other instance (instance which is request for specific data block).  Cache Fusion provides single buffer cache (for all instances in cluster) through interconnect.

In Single Node oracle database, an instance looking for data block first checks in cache, if block is not in cache then goes to disk to pull block from disk to cache and return block to client.

In RAC Database there is remote cache so instance should look not only in local cache (cache local to instance) but on remote cache (cache on remote instance). If cache is available in local cache then it should return data block from local cache; if data block is not in local cache, instead of going to disk it should first go to remote cache (remote instance) to check if block is available in local cache (via interconnect)

This is because accessing data block from remote cache is faster than accessing it from disk.


Cache Fusion Model
Cache fusion Model is dependent on three services
— Global Resource Directory (GRD)
— Global Cache Service (GCS)
— Global En-queue Service (GES) and —

More on Cache Fusion Model coming soon …..
SSH User Equivalency – means assigning same properties (username, userid, group, group id and same password) to operating system user (installing & owning RAC database) across all nodes in cluster

CVU (Cluster Varification Utility) – is utility to verify that system meets all the criteria for Oracle Clusterware Installation.

About the Author Atul Kumar

Oracle ACE, Author, Speaker and Founder of K21 Technologies & K21 Academy : Specialising in Design, Implement, and Trainings.

follow me on:

Leave a Comment:

Ram says December 29, 2008

Hello Atul,

Can you please let us know how to create virtual ip for RAC instance..

Atul Kumar says December 29, 2008

First there should be enough network card in machine and then add VIP by putting them in /etc/hosts (on Unix)

Like loalhost.localdomain localhost rac2.focusthread.com rac2 rac2-vip.focusthread.com rac2-vip rac2-priv.focusthread.com rac2-priv rac1.focusthread.com rac1 rac1-vip.focusthread.com rac1-vip rac1-priv.focusthread.com rac1-priv

as shown in


Ram says December 30, 2008

Hello Atul,

Thanks for your update. We have 2 network cards on each machine and one eth is using for private and another for public. Can you please give me the steps to create vip in both machines. I searched in your blogs and I am unable to find the steps for creating vip.

Thanks in advance

Atul Kumar says January 2, 2009

For Virtual IP just define unused IP in /etc/hosts and give vipname to this .

Do this on second node as well with another IP and ensure that each node can connect to other node using VIP.

Keo Vongkaseum says March 2, 2009


You will need to add the VIP addresses to your DNS server as well. By simply adding entries to /etc/hosts file is not enough…

» Oracle RAC - Clusterware Installation Overview Online Apps DBA: One Stop Shop for Apps DBA’s says June 6, 2009

[…] 1. External Shared Disk to store Oracle Clusterware file (Voting Disk and Oracle Cluster Registry – OCR)  To know more about OCR & Voting Disk click here […]

» Backup and recovery of Oracle Clusterware Online Apps DBA: One Stop Shop for Apps DBA’s says June 9, 2009

[…]  Print This Post The backup of OCR content(Oracle Cluster Registry, to know more about OCR click here) is performed using two […]

prashanth says October 1, 2009

usefull info,


najeeb7 says April 8, 2013

Hi Atul,

I have 1 interview question, if you could answer me. OCSSD is able to send hearbeats to VD but not able to apply read-write lock on OCR, after re-booting both the nodes same issues. How you will solve it…

Debjit Mukherjee says March 4, 2017

hello Atul,

Could you please provide me the good definition of these RAc terminilogy:
1)Cluster Private connect
2)Oracle Clusterware
7)SPLIT Brain

And also if you clear my doubts for Oracle Clusterware,CRS and OCR

Add Your Reply

Not found