CSS Voting Disk Function

• CSS is the service that determines which nodes in the cluster are available and provides cluster group membership and simple locking services to other processes. 
• CSS typically determines node availability via communication through a dedicated private network with a voting disk used as a secondary communication mechanism. This is done by sending heartbeat messages through the network and the voting disk as illustrated by the top graphic in the slide. 
• The voting disk is a file on a clustered file system that is accessible to all nodes in the cluster. 
• Its primary purpose is to help in situations where the private network communication fails. The voting disk is then used to communicate the node state information used to determine which nodes go offline. 
• Without the voting disk, it can be difficult for isolated nodes to determine whether it is experiencing a network failure or whether the other nodes are no longer available. 
• It would then be possible for the cluster to enter a state where multiple subclusters of nodes would have unsynchronized access to the same database files. The bottom graphic illustrates what happens when Node3 can no longer send heartbeats to other members of the cluster. When others can no longer see Node3’s heartbeats, they decide to evict that node by using the voting disk. When Node3 reads the removal message or “kill block,” it generally reboots itself to ensure that all outstanding write I/Os are lost. 
• Oracle Clusterware supports up to 15 redundant voting disks.
• Note: The voting disk or file is usually known as quorum disk in vendor clusterware.