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Oracle RAC DBA - Interview Questions and Answers on Oracle RAC

Date Added: 03 Mar.2019 Date Updated: 10 Oct.2019 Oracle RAC DBA Full Blog

Interview Questions and Answers on Oracle RAC

1.  What is Oracle Real Application Clusters?

Oracle RAC enables you to cluster Oracle databases.Oracle RAC uses Oracle Clusterware for the infrastructure to bind multiple servers so they operate as a single system.Oracle Clusterware is a portable cluster management solution that is integrated with Oracle Database.
2. What are the file storage options provided by Oracle Database for Oracle RAC?
Ans:The file storage options provided by Oracle Database for Oracle RAC are,
Automatic Storage Management (ASM)
OCFS2 and Oracle Cluster File System (OCFS)
A network file system
Raw devices
3.What is RAC and what are the various benefits of using RAC architecture?
Ans: RAC or Real Application Cluster allows the database to be installed across multiple servers forming a cluster and sharing the storage structure at the same time. This prevents the database from a single point of failure as one or the other instance will always stay up even if the other fails.
Using RAC helps in
Maintaining High Availability of the system.
Managing workload with the least expenses.
Scalability & Agility.
4.What do you understand from Cache Fusion?

Ans: Cache fusion is the process of transferring data from one instance buffer cache to another at very high speed within a cluster. Instead of fetching data from physical disk which is a slow process, the data block can be accessed from the cache.

For E.g. Instance A wants to access a data block which is being owned by instance B, it will send an access request to instance B and hence can access the same using the other instance B’s buffer cache.

5. What is split brain?

When database nodes in a cluster are unable to communicate with each other, they may continue to process and modify the data blocks independently. If the same block is modified by more than one instance, synchronization/locking of the data blocks does not take place and blocks may be overwritten by others in the cluster. This state is called split brain.

6. What is a CFS?

A cluster File System (CFS) is a file system that may be accessed (read and write) by all members in a cluster at the same time. This implies that all members of a cluster have the same view.

7. What methods are available to keep the time synchronized on all nodes in the cluster?

Ans. Either the Network Time Protocol(NTP) can be configured or in 11gr2, Cluster Time Synchronization Service (CTSS) can be used.
8. What is Voting Disk?
Voting Disk is a file that maintains node membership details.
All members in the Cluster read and write Heartbeat Information in Voting Disk.
9. What Information is stored in Voting Disk?
Voting disks contain static and dynamic data.
Static data : Info about nodes in the cluster
Dynamic data : Disk heartbeat logging
It maintains and consists of important details about the cluster nodes membership, such as
- which node is part of the cluster,
- which node is joining the cluster, and
- which node is leaving the cluster.
10.  Why do we have odd number of voting disks?
Odd number of Voting disks are required to prevent Split Brain Syndrome.
A node must be able to access more than half of the voting disks in order to decide which node can be evicted incase of failure.
11. Why do we need Voting Disk?
CSSD processes (Cluster Services Synchronization Daemon) monitor the health of  RAC nodes employing two distinct heart beats: Network heart beat and Disk heart beat. Healthy nodes will have continuous network and disk heartbeats exchanged between the  nodes. Break in heart beat indicates a possible error scenario. There are few different scenarios possible 
with missing heart beats:
1. Network heart beat is successful, but disk heart beat is missed.
2. Disk heart beat is successful, but network heart beat is missed.
3. Both heart beats failed.
In addition, with numerous nodes, there are other possible scenarios too. Few possible cenarios:
1. Nodes have split in to N sets of nodes, communicating within the set, but not with members in other set.
2. Just one node is unhealthy.
Nodes with quorum will maintain active membership of the cluster and other node(s) will be fenced/rebooted.
12. How to backup Voting Disk?
Prior to Oracle 11g R2 version, Voting Disk is backed up using dd command
From 11g R2, no need to manually backup voting disk.
It is automatically backup along with OCR  whenever there is a configuration change.
In 11g R2, Restoring Voting disk from a manually copied backupfile may prevent cluster services from starting up
13. What are Nodeapps Services in Oracle RAC?
Nodeapps are standard set of oracle application services which are started automatically for RAC.
Node apps Include:
1) VIP.
2) Oracle Net listener.
3) Global Service Daemon.
4) Oracle Notification Service.
Nodeapp Services run on each node of the cluster and will switched over to other nodes through VIP during the failover.
Useful commands to maintain nodeapps services:
srvctl stop nodeapps -n NODE1 
srvctl stop nodeapps -n NODE2            
srvctl start nodeapps -n NODE1              
srvctl start nodeapps -n NODE2             
srvctl status nodeapps
 14. What are some of the RAC specific parameters?
Some of the RAC parameters are: