Tuesday 27 October 2020

What is Split Brain Syndrome in Oracle RAC?

Split brain syndrome occurs when the Oracle RAC nodes are unable to communicate with each other via private interconnect, but the communication between client and RAC node is maintained. This can cause data Integrity issues  when the same block is read or updated by two nodes and changes done from one node are overwritten by the other node because the block being changed is not locked.

When a node fails, the failed node is prevented from accessing all the shared disk devices and groups. This methodology is called I/O Fencing, Disk Fencing or Failure Fencing.

The node which first detects that one of the node is not accessible will evict that node from the RAC cluster group.This problem is solved by configuring the heartbeat connections through the same communication channels that are used to access the clients.

What causes Node eviction in Oracle RAC?

What is node eviction?

Node eviction in RAC is done when a heartbeat indicates that a node is not responding, 

and the evicted node is re-started to make it a part of cluster.

Causes for RAC node eviction:

Node eviction on Oracle RAC environment can be due to any of the below reasons. 

- A failure of any of the major hardware components (CPU, RAM, network interconnect).

- A server that is experiencing RAM swapping.

- When communications to the voting disk is interrupted, causing the disconnected node to be evicted and re-boot.

- Database or ASM hang condition.

Below is the list of important log files to review in case of a node eviction

- Clusterware alert log

- Database alert log

- CSSD agent logs

- CSSD monitor logs

- System Message logs (/var/log/messages)

What is Flashback feature in Oracle Database?

Oracle Flashback Database and restore points are related data protection features that enable you to rewind data back in time to correct any problems caused by logical data corruption or user errors within a designated time window.

Points to Remember

- Flashback Database command can be run either from SQLPLUS or using RMAN Utility.

- FLASHBACK DATABASE command can be used to rewind the database to a target time, SCN or a log sequence number.

- Flashback command works by undoing the changes made to the data files that exist when you run the command.

- Flashback can fix only logical failures, not physical failures.

- If the database control file is restored from backup or re-created, then all existing flashback log information is discarded.

- Avoid using FLASHBACK DATABASE with a target time or SCN that coincides with a NOLOGGING operation, it can cause block corruption.

How to enable Flashback Database feature in Oracle?

 Prerequisites for Flashback Database and Guaranteed Restore Points

Flashback Database

Configure the following database settings before enabling Flashback Database:

-Your database must be running in ARCHIVELOG mode.

-You must have a fast recovery area enabled.

-For Oracle Real Application Clusters (Oracle RAC) databases, the fast recovery area must be in a clustered file system or in ASM.

Guaranteed Restore Points

To use guaranteed restore points,the COMPATIBLE initialization parameter must be set to 10.2.0 or greater.

Steps to enable Flashback Database:

1. Connect to sqlplus as sysdba and set  the desired value for Flashback retention target using below command.


Here, Flashback retention target is set to window of 3 days(4320 Minutes), default value is 1 day (1440 Minutes).

2. Enable the Flashback Database feature for the whole database using the following command:


3.Use the following command to check if Flashback Database is enabled for your target database: