Real Application Cluster(RAC) Interview Questions Part 2

1. What is RAC?

• RAC stands for Real Application cluster.
• It is a clustering solution from Oracle Corporation that ensures high availability of databases by providing instance failover, media failover features.

2.How is it different from non RAC databases?

• Cluster is the key component and is a collection of servers operations as one unit.
• RAC is the best solution for high performance and high availability.
• Non-RAC databases has single point of failure in case of hardware failure or server crash.

3.What is cache fusion?

• Cache fusion is nothing but a mapping of remote memory of oracle buffers, which is shared between the caches participating nodes in the cluster.
• It is very easy to gain the block image from the instance that contain the block in its SGA instead of reading from the disk
• This happens when the block of data is read from data file by an instance in the cluster and when another instance require the same block.

4.What is the purpose of OLR?

Oracle Local Repository (OLR) contains an information which allows the cluster programs to initiate with the OCR, which is being in the ASM storage. As until the grid processes are started,  the ASM file is unavailable, then a local copy of the data of the OCR is required, that is stored in OLR.

5.What is FAN?

• FAN stands for Fast Application Notification, which is connected to the events containing services, nodes and instances.
• In order to describe the other processes about the service level information and configuration which contains the changes of the service status like UP or DOWN events.
•  Oracle RAC 12c uses this notification mechanism.
• Using FAN events, the application gives response and can take immediate actions.

6.What is the difference between instance and crash recovery?

• The crash recovery takes place during the startup, when an instance breaks up in a single node database.
• If the same recovery is performed in the RAC environment by the surviving nodes,  it is called as an instance recovery.

7.What is hangcheck timer?

The hangcheck timer is used to check the health of the system regularly. The Node is restarted automatically, when the system stops or hangs.

There are two key parameters:

Hangcheck Margin:  This shows that how much delay can be permitted before the reset of the RAC node is done by the hangcheck timer.

Hangcheck Tick: It is defined as the time period between system health checks. 60 seconds is the default time, but Oracle recommends it to be 30 seconds.

8.What is GRD?

GRD is the Global Resource Directory. The GRD is used by the GES and GCS to maintain the records of each cached block and each datafile. This process is known as cache fusion and can be used in data integrity.

9.What is OCR file?

• It is a RAC configuration information repository, which maintains the information about the instance node mapping and cluster node.
• It maintains information about the  profiles of Oracle Clusterware Resource for customed applications.
• It manages the configuration information related to any cluster database in the cluster.
• It is necessary for the OCR to reside on a shared disk, which is accessible by all of the cluster nodes.
• The command daemon OCSSD maintains the configuration information in OCR and manages the changes to cluster within the registry.

10.What is a raw partition?

It is the part of the physical disk, which is accessed on the lowest level. When an additional partition is created, raw partition is created and without any formatting, a logical partitions are assigned to it. It is called cooked partition, once the formatting is completed.

11.What is load balancing Advisory?

To check the workload across resources in the balancing of application, the load balancing advisory is provided.

12.How do I enable the Load Balancing Advisory (LBA)?

To enable the load balancing advisory, use the ‘-B’ option when creating or modifying the service using the ‘srvctl’ command.

13.What options are required to resolve OCR, if it is corrupted?

The backup copy of either physical or logical OCR copy is used to restore the repository.

14. What is a voting disk?

A voting disk is a file that manages information about node membership.

15. What are the administrative tasks involved with voting disk?

Following administrative tasks are performed with the voting disk :

1. Backing up voting disks
2. Recovering Voting disks
3. Adding voting disks
4. Deleting voting disks
5. Moving voting disks

16.How many voting disks are you maintaining ?

• By default Oracle will create 3 voting disk files in ASM.
• Oracle expects that you will configure at least 3 voting disks for redundancy purposes.
• You should always configure an odd number of voting disks >= 3.
• This is because loss of more than half your voting disks will cause the entire cluster to fail.
• You should plan on allocating 280MB for each voting disk file.
• If you are using ASM and normal redundancy you will need 560MB.

17. Mention the Oracle RAC software components ?

Oracle RAC is composed of two or more database instances.

They are composed of Memory structures and background processes same as the single instance database.Oracle RAC instances use two processes GES(Global Enqueue Service), GCS(Global Cache Service) that enable cache fusion. Oracle RAC instances are composed of following background processes:

ACMS—Atomic Controlfile to Memory Service (ACMS)

GTX0-j—Global Transaction Process

LMON—Global Enqueue Service Monitor

LMD—Global Enqueue Service Daemon

LMS—Global Cache Service Process

LCK0—Instance Enqueue Process

RMSn—Oracle RAC Management Processes (RMSn)

RSMN—Remote Slave Monitor

18.What are SCAN components in a cluster?

• SCAN Name
• SCAN IPs (3)
• SCAN Listeners (3)

19.What is TAF and TAF policies?

• Transparent Application Failover (TAF)  is runtime failover for high availability environments, such as Real Application Clusters and Oracle Real Application Clusters Guard.
•  TAF refers to the failover and re-establishment of application-to-service connections.
• It enables client applications to automatically reconnect to the database if the connection fails, and optionally resume a SELECT statement that was in progress.
• This reconnect happens automatically from within the Oracle Call Interface (OCI) library.

20.Which enable the load balancing of applications in RAC?

Oracle Net Services enable the load balancing of application connections across all of the instances in an Oracle RAC database.