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Real Application Cluster(RAC) Interview Questions Part 1

1.What are the types of Oracle Cluster-Ware process for 10g on Unix/Linux?

 Cluster Synchronization Services (ocssd)

Manages cluster node membership and runs as the oracle user, failure of this process results in Cluster restart.

Cluster Ready Services (crsd)

The Cluster Ready Services process manages cluster resources (that could be a Database, an Instance, Service,  Listener, Virtual IP (VIP) address, an Application Process, and So On) based on the resource’s configuration information that is stored in the OCR. This includes Start, Stop, Monitor and Fail-over operations. This process runs as the Root user

Event manager daemon (evmd)

A background process that publishes events that CRS creates.

Process Monitor Daemon (OPROCD)

  • This process monitor the cluster and provide I/O fencing.
  • OPROCD perform its check, stops running, and if  the wake up is beyond the expected time, then OPROCD resets the processor and reboots the node.
  • An OPROCD failure results in Oracle Cluster-ware restarting the node.
  • OPROCD uses the hangcheck timer on Linux platforms.

RACG (racgmain, racgimon)

It Extends cluster-ware to support Oracle-specific requirements and complex resources. Runs server call-out scripts when FAN events occur.

 

2.What are Oracle database background processes specific to Real Application Cluster (RAC)?

LMSGlobal Cache Service Process

LMDGlobal Enqueue Service Daemon

LMONGlobal Enqueue Service Monitor

LCK0Instance Enqueue Process

Oracle RAC instances use two processes, the Global Cache Service (GCS) and the Global Enqueue Service (GES).

The GCS and GES records the statuses of each data file and each cached block using a Global Resource Directory (GRD).

The GRD contents are distributed across all of the active instances.

 

3.What are Oracle Cluster-ware Components?

 

Voting Disk

  • Oracle RAC uses the voting disk to manage cluster membership by way of a health check and arbitrates cluster ownership among the instances in case of network failures.
  • The voting disk must reside on shared disk.

Oracle Cluster Registry (OCR)

  • It Maintains cluster configuration information as well as configuration information about any cluster database within the cluster.
  • The OCR must reside on shared disk that is accessible by all of the nodes in your cluster.

 

4. How would you troubleshoot for node reboot ?

For Detail Answer Please check Metalink  Notes

  • Note 265769.1 Troubleshooting CRS Reboots
  • Note.559365.1 Using Diagwait as a diagnostic to get more information for diagnosing Oracle Clusterware Node evictions.

 

5) How would you take backup of  OCR ?

There is an automatic backup mechanism for OCR. The default location is : $ORA_CRS_HOME\cdata\”clustername”\

To display backups :

#ocrconfig -showbackup

To restore a backup :

#ocrconfig -restore

With Oracle RAC 10g Release 2 or later, you can also use the export command:

#ocrconfig -export -s online, and use -import option to restore the contents back.

With Oracle RAC 11g Release 1, you can do a manaual backup of the OCR with the command:

# ocrconfig -manualbackup

 

6. How would you take backup of  voting disk ?

#dd if=voting_disk_name of=backup_file_name

 

7) How do I identify the voting disk location ?

#crsctl query css votedisk

 

8) How do I identify the OCR file location ?

check /var/opt/oracle/ocr.loc or /etc/ocr.loc ( depends upon platform)

or

#ocrcheck

 

9) Is ssh required for normal Oracle RAC operation ?

“ssh” are not required for normal Oracle RAC operation. However, it is recommended that “ssh” should be enabled for Oracle RAC and patchset installation.

 

10) What is SCAN?

Single Client Access Name (SCAN) is s a new Oracle Real Application Clusters (RAC) 11g Release 2 feature that provides a single name for clients to access an Oracle Database running in a cluster. The benefit is clients using SCAN do not need to change if you add or remove nodes in the cluster.

11) What is the purpose of Private Interconnect ?

  • Clusterware uses the private interconnect for cluster synchronization (network heartbeat) and daemon communication between the the clustered nodes. This communication is based on the TCP protocol.
  • RAC uses the interconnect for cache fusion (UDP) and inter-process communication (TCP).
  • Cache Fusion is the remote memory mapping of Oracle buffers, shared between the caches of participating nodes in the cluster.

12.What is a virtual IP address or VIP?

  • A virtual IP address or VIP is an alternate IP address that the client connections use instead of the standard public IP address.
  • To configure VIP address, we need to reserve a spare IP address for each node, and the IP addresses must use the same subnet as the public network.

 

13.What is the use of VIP?

If a node fails, then the node’s VIP address fails over to another node on which the VIP address can accept TCP connections but it cannot accept Oracle connections.

14.Give situations under which VIP address failover happens?

VIP addresses fail-over happens when the node on which the VIP address runs fails, all interfaces for the VIP address fails, all interfaces for the VIP address are disconnected from the network.
Using virtual IP we can save our TCP/IP timeout problem because Oracle notification service maintains communication between each nodes and listeners.

 

15.What is the significance of VIP address failover?

When a VIP address failover happens, Clients that attempt to connect to the VIP address receives  rapid connection refused error .They don’t have to wait for TCP connection timeout messages.

 

13) What do you do if you see GC CR BLOCK LOST in top 5 Timed Events in AWR Report?

This is most likely due to a fault in interconnect network.

Check netstat -s

if you see “fragments dropped” or “packet re-assemblies failed” , Work with your System Administrator to find the fault with network.

 

14.How many nodes are supported in a RAC Database?

In 10g Release 2, RAC support 100 nodes in a cluster using Oracle Clusterware, and 100 instances in a RAC database.

 

15.Suppose, Srvctl is not starting the instance, generating the following error PRKP-1001 CRS-0215, however sqlplus can start it on both nodes? How would you identify the problem?

  • Set the environmental variable SRVM_TRACE to true.
  • Start the instance with srvctl.
  • Now we will get detailed error stack.

 

16. What is the purpose of the Oracle Notification Service (ONS) daemon?

The Oracle Notification Service (ONS) daemon is an daemon started by the CRS clusterware as part of the nodeapps. There is one ONS daemon started Per Clustered Node.

The ONS daemon receive a subset of published clusterware events via the local evmd and racgimon clusterware daemons and forward those events to application subscribers and to the local listeners.

 

17. What is the split-brain scenario?

In Oracle RAC, split-brain is the scenario when one or more nodes updates to the database files w/o considering the integrity with other nodes. So, in that scenario there is high possiblity of compromissing of database integrity and introducing the corruption to the database.

 

About Syed Saad Ali

As a certified and experienced Oracle Database Administrator with Almost 10 Years of Experience I am able to perform all levels of database maintenance and am fully qualified to do Oracle updates as well. Capitalizing on my interest in Business & IT both, I chose to navigate my career towards Enterprise Solutions, which has enabled me to excel in the combination of both. I have developed exceptional analytical skills that allow me to diagnose and take care of issues immediately. Extremely skilled in planning and execution of special projects in time- critical environments. Extensive knowledge in directing, coordinating and exercising concern authority for planning, organization, control and successful completions. Very skilled at solving customer relations issues by seeking first to understand and then to be understood. Effectively able to communicate with customers, vendors and management.

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