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.
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[PS] C:\>Get-Cluster | Get-ClusterNode Name State ---- ----- ho-ex2010-mb1 Up ho-ex2010-mb2 Up [PS] C:\>Get-MailboxDatabase | Test-MAPIConnectivity MailboxServer Database Result Error ------------- -------- ------ ----- HO-EX2010-MB1 MB-HO-01 Success HO-EX2010-MB1 MB-HO-02 Success BR-EX2010-MB MB-BR-01 Success HO-EX2010-MB1 MB-HO-03 Success HO-EX2010-PF MB-HO-Archive Success |
Next, I take down the file share witness, and then a short time later one of the DAG members as well.
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[PS] C:\>Get-Cluster | Get-ClusterNode Name State ---- ----- ho-ex2010-mb1 Up ho-ex2010-mb2 Down |
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.
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[PS] C:\>Get-MailboxDatabase | Test-MAPIConnectivity MailboxServer Database Result Error ------------- -------- ------ ----- HO-EX2010-MB1 MB-HO-01 *FAILURE* Database is dismounted. HO-EX2010-MB1 MB-HO-02 *FAILURE* Database is dismounted. BR-EX2010-MB MB-BR-01 Success HO-EX2010-MB1 MB-HO-03 *FAILURE* Database is dismounted. HO-EX2010-PF MB-HO-Archive Success |
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.
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[PS] C:\>Get-MailboxDatabase | Test-MAPIConnectivity Creating a new session for implicit remoting of "Get- MailboxServer Database Result Error ------------- -------- ------ ----- E15MB1 Mailbox Database 1 Success E15MB1 Mailbox Database 2 Success [PS] C:\>Get-Cluster | Get-ClusterNode | Select Name,DynamicWeight,NodeWeight,Id,State Name DynamicWeight NodeWeight Id State ---- ------------- ---------- -- ----- E15MB1 1 1 1 Up E15MB2 1 1 2 Up E15MB3 1 1 3 Up |
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.
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[PS] C:\>Get-Cluster | Get-ClusterNode | Select Name,DynamicWeight,NodeWeight,Id,State Name DynamicWeight NodeWeight Id State ---- ------------- ---------- -- ----- E15MB1 0 1 1 Up E15MB2 1 1 2 Up E15MB3 0 1 3 Down |
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.
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[PS] C:\>Get-Cluster | Get-ClusterNode | Select Name,DynamicWeight,NodeWeight,Id,State Name DynamicWeight NodeWeight Id State ---- ------------- ---------- -- ----- E15MB1 1 1 1 Up E15MB2 0 1 2 Down E15MB3 0 1 3 Down [PS] C:\>Get-MailboxDatabase | Test-MAPIConnectivity MailboxServer Database Result Error ------------- -------- ------ ----- E15MB1 Mailbox Database 1 Success E15MB1 Mailbox Database 2 Success |
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:
