Cloud Witness is a new type of Failover Cluster quorum witness that will be available in the next version of Windows Server – now available in Technical Preview. The idea is to use the Azure Storage to place the cluster witness.
Cluster uses the Azure Blob Storage for a blob file which gets a vote and participates in quorum calculations and the same storage account can be used for multiple clusters. In the case you are worried about the expenses keep in mind that the Azure storage is billed based on data amount stored and that a witness blob file is very small – insignificant in size to be more precise.
The same storage account used by two clusters:
There are two main scenarios when you should use the Cloud Witness:
- Stretched clusters
- Clusters running within Azure VMs
When having a stretched cluster it is important to ensure that nodes in a secondary location can failover roles. Imagine that you have a 4 node failover cluster with 2 nodes at each location. Each node has one vote, but we need an extra vote to be able to calculate the quorum and determine which part of the cluster will continue to serve clients. For a stretched clusters we couldn’t use the locally shared disk – as for a local clusters -, we have to use a file share. Placing this file share was an issue and for the proper setup it has to be in the third location and it has to be highly available. Having a third location just for placing witness was demanding and very often it is placed in one of the two locations. In the case of primary location disaster and file share being placed there we would loose an entire cluster. Two nodes in the second location can not tell if they just lost connection to primary site or there was a disaster. They consist only two votes, while the other part of the cluster has three votes (2 nodes and witness) and the nodes on the second location will not failover roles from the nodes on the primary location. Solution is to place the witness in a third location – in the Azure as a highly available platform.
Clusters running within Azure VMs do not have a shared storage and therefore we can not use a shared disk as a cluster witness. So far we had to use a file share, but that required an extra VM just to place the share. Now we can use the Azure storage account as already mentioned which makes building virtual clusters in Azure less complicated.
Configuring the Cloud Witness is very simple, just follow the steps below.
Create Storage Account in Azure and get Manage Keys
Make sure you choose Locally Redundant replication since Geo Replicated can not guarantee consistency for blob file.
Storage Account Name and Access Key you will need to setup cluster quorum. If you want to regenerate the primary key for example, first update all your clusters with secondary key and vice verse.
Configure Cloud Witness