Why do we need voting Disk or Voting File

Question

Hi all
What is the need of Voting file or disk??? i know it maintains the node information n all. But if nodes can ping all other nodes what is the need of voting disk.
Can anyone please explain.
PS : I read the
Oracle® Database
Oracle Clusterware and Oracle Real Application Clusters
Administration and Deployment Guide
10g Release 2 (10.2)
But could not find how does it actually come into picture.
Thanks in anticipation

Answer 1

Lets take the case of a 2 node RAC - instance 'A' and instance 'B';
Lets assume that instances A & instance B is unable to communicate with each other. Now, this might be due the fact that the other instance is actually down oR because there is some problem with the interconnect communication between the instances.
A split-brain occurs when the instances cannot speak to each other due to interconnect communication problem and each instance happens to think that it's the only surviving instance and starts updating the database. This obviously is problematic, isn't it?
To avoid split brain, both instances are required to update the voting disk periodically.
Now consider the earlier case from the perspecive of one of the nodes:
case 1) instance B is actually down:- Here, looking from instance A's perspective; there is no communication over the interconnect with instance B and also instance B is not updating the voting disk ('coz it's down). Instance A can safely assume that instance B is down and it can go on providing the services and updating the database.
Case 2) the problem is with interconnect communication:- Here, looking from instance A's perspective; there is no communication over the interconnect with instance B. However it can see that instance B is updating the voting disk; which means instance B is not actually down. This state is true from instance B's perspective; it can see that instance A is updating the voting disk but it cannot speak to instance A over the interconnect. At this point, both instances rush and try to lock down the voting disk and whoever gets the lock ejects the other one thus avoiding split-brain syndrome.
That's the theory behind using a voting disk.
When the instances of a RAC boots up, one of the instances will be assigned the master. In the case of a split-brain the master gets the lock on the voting disk and survives. In reality the instance with the lower node number survives, even if the problem is with the interconnect network card on that node :)
Cheers.