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Top 16 Oracle RAC Interview Questions

1.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.
2.What are the components of clusterware?
Oracle cluster registry (OCR): It contains all information about instances, services, state information, cluster configuration, nodes and ASM storage if needed.  The OCR should occupy on a shared disk, which is accessible by all the nodes in your cluster. In OCR, the daemon OCSSd is used to manage the configuration and in the registry, it maintains the changes to the cluster.
Voting Disk: It helps to verify, if a node has failed, which means it got separated from the majority, then it is rebooted forcibly and after rebooting,  it is added again to the surviving nodes of cluster. The Oracle RAC uses it to maintain the membership of cluster.
3.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.
4.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.
5.What is SCAN?
SCAN stands for Single Client Access Name is a feature of new Oracle RAC 11g release 2 which provides one name for clients to access an Oracle Database cluster. The benefit to the SCAN user is that, there is no need to change if you remove or add nodes in the cluster.
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. The same recovery is performed in the RAC environment by the surviving nodes,  which 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. They are:
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 the split brain?
When nodes of the database in a cluster can’t communicate with each other, they modify the data blocks and may continue to process independently. If more than one instance modify the same block, locking or synchronization of the blocks of the data does  not occur and it may happen that the blocks get overwritten by others in the cluster. This process is called split brain.
9.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.
10.What is OCR file?
It is a RAC configuration information repository, which maintains the information about the instance node mapping and cluster node. It also 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.
11.What is an interconnect network?
It is a private network, which connects all the servers in a cluster. It uses the multiple switches which are accessed by only the nodes in the cluster.
12.What is a raw partition?
It is the part of the physical disk, which is accessed on the lowest level. When an addition 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.
13.What is the use of VIP?
When the node fails, then the VIP address of that node fails over to the other node on which it cannot  accept Oracle connections but not TCP connections.
14.What is load balancing Advisory?
To check the workload across resources in the balancing of application, the load balancing advisory is provided.
15.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.


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