Voting Disks in RAC

Voting Disks in RAC

Voting disks are important component of Oracle Clusterware. Voting disk is a file that manages information about node membership.It is located on the shared cluster system or a shared raw device file or ASM (11gr2 onwards) .Its primary purpose is to help in a situation where the private network communication fails. CSS (Cluster Synchronization Service) is the service that determines which nodes in the cluster are available via communication through a dedicated private network and with a voting disk used as a secondary communication mechanism. 

Important points about Voting disks in RAC

These files can be stored either in ASM or on shared storage.

1)If it is stored in ASM, no need to configure manually as the files will be created depending on the redundancy in ASM.

2) In shared storage system, we need to manually configure these files with redundancy setup for high availability.

a) We must have odd number of disks.

b)Oracle recommends minimum of 3 and maximum of 5. In 10g, Clusterware can supports 32 voting disks but in 11gR2 supports 15 voting disks.

c)A node must be able to access more than half of the voting disks at any time. For eg, if you have 5 voting disks, a node must be able access atleast 3 of the voting disks. If it cannot access the minimum of voting disks, then it is evicted/removed from the cluster.

d) All nodes in the RAC cluster register their heartbeat information in the voting disks/files. RAC heartbeat is the polling mechanism that is sent over the cluster interconnect to ensure all RAC nodes are available.

What is stored in voting disks in RAC?

Voting disks contain static and dynamic data.

Static data : Info about nodes in the cluster

Dynamic data : Disk heartbeat logging

It maintains and consists of important details about the cluster nodes membership, such as
– which node is part of the cluster,
– who (node) is joining the cluster, and
– who (node) is leaving the cluster.

Important Operation for Voting disks in RAC

a) To list currently configured voting disk
$ORA_CRS_HOME/bin/crsctl query css votedisk

b) Adding or deleting the Voting disks

crsctl add css votedisk

Run the following command as the root user to remove a voting disk:

crsctl delete css votedisk

c)  replacing Voting disk

crsctl replace votedisk

Voting disk backup  (In 10g)

dd if=<voting-disk-path> of=<backup/path>

Voting disk restore (In 10g)

dd  if=<backup/path>  of=<voting disk path>

In 11gR2, the voting files are backed up automatically as part of OCR.  Oracle recommends NOT used dd command to backup or restore as this can lead to loss of the voting disk.

Add/delete vote disk

crsctl add css votedisk <path> -adds a new voting disk

crsctl delete css votedisk <path> -- deletes the voting disk