Imagine a multi-site or stretch cluster that spans sites A and B. Hosts are running in Site A and Site B and are nodes in that cluster. A critical function of a cluster is to calculate quorum in the event of a site outage or a link failure – one site must have more than half of the available votes to have quorum and continue operating as a cluster (and failover the VMs from the other site).
The recommended solution for this in the past was to create a file share witness … operating in a third site. So first you needed a third site. That’s not going to be cheap! And then you needed, ideally, a file server cluster running in that third site to ensure that the file share witness was highly available.
Windows Server vNext offers a new advanced quorum option of using a Cloud Witness. This is where we use Azure as a witness. Folks – this is going to be one seriously affordable option!
You create a blob storage account in Azure. This will store just an incremental sequence number; it’s just a vote so the cluster will remain operational if Azure has issues. This will cost just a few cents per month to operate.
You then configure the cluster with the storage account name and storage account key for secure access. The cluster will then use Azure as a witness. This is a lot easier and cheaper than configuring a file share witness on a cluster in a third site.