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Improving Resilience of Exchange Server 2013 Database Availability Groups with Windows Server 2012 Cluster Dynamic Quorum

Exchange Server 2013 can be installed on either Windows Server 2008 R2 or Windows Server 2012.

Some organizations may decide to install on Windows Server 2008 R2 because that is their standard server build and to remain consistent with the rest of their server fleet. However, doing that will mean they miss out on the new features of Windows Server 2012.

One of those new features is a cluster quorum management option known as dynamic quorum.

As TechNet explains:

When this option is enabled, the cluster dynamically manages the vote assignment to nodes, based on the state of each node. Votes are automatically removed from nodes that leave active cluster membership, and a vote is automatically assigned when a node rejoins the cluster.

With dynamic quorum management, it is also possible for a cluster to run on the last surviving cluster node. By dynamically adjusting the quorum majority requirement, the cluster can sustain sequential node shutdowns to a single node.

In an Exchange context, dynamic quorum can make database availability groups more resilient to multiple node failures.

To demonstrate this, here is what happens to an Exchange Server 2010 DAG when it suffers multiple node failures.

To begin with the DAG is healthy and all nodes and resources are online.

exchange-2010-dag-quorum-02

Next, I take down the file share witness, and then a short time later one of the DAG members as well.

exchange-2010-dag-quorum-01

After a few moments the cluster determines that quorum has been lost, and the remaining node stops as well.

Log Name: System
Source: Microsoft-Windows-FailoverClustering
Date: 5/27/2013 8:12:22 PM
Event ID: 1177
Task Category: Quorum Manager
Level: Critical
Keywords:
User: SYSTEM
Computer: HO-EX2010-MB1.exchangeserverpro.net
Description:
The Cluster service is shutting down because quorum was lost. This could be due to the loss of network connectivity between some or all nodes in the cluster, or a failover of the witness disk.

The entire cluster is now down, taking the mailbox databases with it, even though a single DAG member was still online.

Now let's take a look at what happens to an Exchange 2013 DAG running on Windows Server 2012 with dynamic quorum enabled (which is the default setting). This Exchange 2013 DAG in my lab happens to have more Mailbox servers as members than my Exchange 2010 DAG, but that does not impact the demonstration.

Again, to begin with the cluster resources are all healthy and online.

exchange-2013-dag-quorum-01

Each node currently has 1 vote (shown as DynamicWeight in the output above). Two of three votes (a majority) is required to achieve quorum, which the cluster has.

First I'll shut down one of the DAG members. Now let's take another look at the nodes.

As this article explains, dynamic quorum kicks in and removes the vote from one of the remaining cluster nodes. Now only one node has a vote, and quorum is maintained.

If this were a Windows Server 2008 R2 cluster quorum would also be maintained, however the difference is in what happens on the next node failure.

Next I take down another DAG member. With one remaining DAG member the Exchange 2010 cluster and databases went offline. However the Exchange 2013 DAG stays online thanks to dynamic quorum.

While this is only a simple demonstration it does show the potential of dynamic quorum for making Exchange 2013 database availability groups more resilient.

Although there are other failure scenarios that may still cause the DAG to go offline (eg multiple simultaneous server failures), with the right cluster design and operational procedures for managing the cluster you can achieve a good outcome.

For more on dynamic quorum in Windows Server 2012:

Paul is a Microsoft MVP for Office Servers and Services. He works as a consultant, writer, and trainer specializing in Office 365 and Exchange Server. Paul is a co-author of Office 365 for IT Pros and several other books, and is also a Pluralsight author.

Exchange Server 2007 CCR recommendations

From Tales of a System Administrator come these recommendations for Exchange 2007 CCR deployments:

 

Having had Cluster Continuous Replication in place now for about a year, I’ve derived some of my own best practice recommendations that might not be found within the official Microsoft documentation.

Although these recommendations are based on a very solid and stable production environment…anything bad or job search inducing that happens as a result of implementing them in your environment is not my responsibility.

Read the rest…

Link: My “unofficial” Exchange Server 2007 CCR recommendations

Paul is a Microsoft MVP for Office Servers and Services. He works as a consultant, writer, and trainer specializing in Office 365 and Exchange Server. Paul is a co-author of Office 365 for IT Pros and several other books, and is also a Pluralsight author.

Video Series: Exchange Server 2007 SP1 Standby Continuous Replication

Scott Schnoll of the Microsoft Exchange Team has released a five part video series on the new Standby Continuous Replication (SCR) feature of Exchange Server 2007 SP1.

The videos are available for viewing or download here.

Read more about SCR on Technet.

Paul is a Microsoft MVP for Office Servers and Services. He works as a consultant, writer, and trainer specializing in Office 365 and Exchange Server. Paul is a co-author of Office 365 for IT Pros and several other books, and is also a Pluralsight author.