Windows Server 2012

What is Windows Server 2012 R2 Dynamic Quorum?

In “Windows Server Failover Clustering: Why Cluster Quorum Matters,” I explained how a Windows Server Failover Cluster uses quorum to decide if or how the cluster should remain operational if it is partitioned by a network problem. The mechanism that was most commonly used to provide quorum was rather rigid and required a lot of care in a rapidly growing environment. In this article, I’ll describe what Windows Server 2012 R2 dynamic quorum is and give you my thoughts on the best way to approach it.

Microsoft has placed a lot of focus on such clouds in Windows 2012 R2 and System Center, and it should be no surprise that one of the new features in Windows Server 2012 R2 (WS2012) Failover Clustering is designed to simplify quorum management.

What is Quorum?

A cluster is made up of several different nodes. In the case of Hyper-V, each node is a Hyper-V host, each using common storage to share virtual machine files, and each connected by networks. A NetFT driver uses these networks to enable a heartbeat signal to be transmitted between the nodes. If the cluster becomes partitioned — in other words, if there is a network failure that divides the cluster into isolated non-communicating islands — then the cluster needs a way to decide which partition should remain active and host services, and which partitions should temporarily drop out of cluster services until the problem is resolved.

If we had an uneven number of nodes, then the split is likely to be 2:1. The partition with two nodes has more than half of the cluster’s nodes and will remain operational. The other partition does not have more than half of the nodes and will failover its HA roles to the nodes in the winning partition.

Sponsored Content

What is “Inside Microsoft Teams”?

“Inside Microsoft Teams” is a webcast series, now in Season 4 for IT pros hosted by Microsoft Product Manager, Stephen Rose. Stephen & his guests comprised of customers, partners, and real-world experts share best practices of planning, deploying, adopting, managing, and securing Teams. You can watch any episode at your convenience, find resources, blogs, reviews of accessories certified for Teams, bonus clips, and information regarding upcoming live broadcasts. Our next episode, “Polaris Inc., and Microsoft Teams- Reinventing how we work and play” will be airing on Oct. 28th from 10-11am PST.

Uneven number of nodes achieving quorum

Uneven number of nodes achieving quorum. (Image: Aidan Finn)

If there are an even number of nodes, then there is a chance that the cluster will partition evenly, maybe two nodes in one partition and two nodes in the other partition. If that happens, then neither side has more than half of the nodes and cannot achieve quorum. There is a tie, and both partitions drop out of operation.

Even number of nodes achieving quorum

Even number of nodes achieving quorum. (Image: Aidan Finn)

Obviously that is bad. The traditional solution is to install a disk witness if there’s an even number of nodes. This quorum disk is used to break the vote. However, if there were an uneven number of nodes, then we were advised to remove the quorum disk. That meant that some poor sucker was frequently adding and removing disks in rapidly growing environments, such as a public cloud or huge private cloud.

Dynamic Quorum

This new feature, also called Dynamic Witness, was added in Windows Server 2012 R2 Failover Clustering to deal with a situation where the quorum requirements of a cluster are changing frequently. By default, every node in a cluster gets a vote for deciding quorum. Dynamic Quorum is automatically used by Failover Clustering to rig the vote in case there is a chance of a tied vote, where a vote will be stripped from one of the nodes.

You can see in the following example that both Demo-Host1 and Demo-Host2 each have an assigned vote. However, there is no file share witness or witness disk and this leads to a tied vote if this cluster was partitioned. Dynamic Quorum fixes the vote by removing a current vote from Demo-Host2. Therefore, if this cluster becomes partitioned, Demo-Host2 will temporarily cease cluster operations until it can communicate with Demo-Host1 once again.

Windows Server 2012 R2 Dynamic Quorum

Windows Server 2012 R2 Dynamic Quorum. (Image: Aidan Finn)

And there’s the catch! What if I perform scheduled maintenance that brings Demo-Host1 offline? I can tell you from experience that the virtual machines on Demo-Host1 will not failover to Demo-Host2. Demo-Host2 cannot vote, and therefore it will not achieve quorum to continue cluster operations.

This is why Microsoft advises to always use a disk or file share witness when deploying Windows Server 2012 R2 clusters. Dynamic Clustering will be used to adjust the vote if there are an uneven number of nodes, and reset the vote to default if there’s an even number of nodes.

Related Topics:

Aidan Finn, Microsoft Most Valuable Professional (MVP), has been working in IT since 1996. He has worked as a consultant and administrator for the likes of Innofactor Norway, Amdahl DMR, Fujitsu, Barclays and Hypo Real Estate Bank International where he dealt with large and complex IT infrastructures and MicroWarehouse Ltd. where he worked with Microsoft partners in the small/medium business space.
External Sharing and Guest User Access in Microsoft 365 and Teams

This eBook will dive into policy considerations you need to make when creating and managing guest user access to your Teams network, as well as the different layers of guest access and the common challenges that accompany a more complicated Microsoft 365 infrastructure.

You will learn:

  • Who should be allowed to be invited as a guest?
  • What type of guests should be able to access files in SharePoint and OneDrive?
  • How should guests be offboarded?
  • How should you determine who has access to sensitive information in your environment?

Sponsored by: