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Microsoft Multi-site Cluster Users Rejoice

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It Is Now Possible To Have Automatic Failover In A 3 Node Cluster!

Microsoft recently released a patch that allows you to specify whether or not a cluster node can vote in in a majority quorum model. This is particularly useful in a multisite cluster configuration that consists of an even number of nodes.

http://support.microsoft.com/kb/2494036

Consider the following…

I have a two node cluster in a local site high availability and I wish to extend it to a 3rdlocation and add a single node for disaster recovery. Sound like a great plan as a multisite cluster is just about the most robust DR plan you can implement. However, you will not be able to take advantage of one of the best features of a multisite cluster – automatic recovery in the event of a site loss. If you were to lose your primary site the DR site only contains one cluster node (see Figure 1). This is just one vote out of three in the cluster so a majority cannot be obtained and Node3 will not come online automatically. The only way to make Node3 come online is to force the quorum online, which kind of defeats the purpose of multisite cluster by requiring human intervention for a failover to happen.

Figure 1 – In a typical 3 node multisite cluster if you lose the primary site the DR site cannot obtain majority so failover never occurs.
Figure 1 – In a typical 3 node multisite cluster if you lose the primary site the DR site cannot obtain majority so failover never occurs.

The only “safe” way to have automatic failover in a multisite cluster is to have an equal number of nodes in each site and to have a file share witness in a 3rd location with connectivity back to both the primary site and the DR site. This concept is a little difficult to grasp at first, so let me attempt to explain through illustrations.

Figure 2- With an even number of nodes in both locations and the file share witness in the primary site a loss of the primary site would not result in a failover as the Alternate Site would only have 2 out of 5 votes, not a majority.
Figure 2- With an even number of nodes in both locations and the file share witness in the primary site a loss of the primary site would not result in a failover as the Alternate Site would only have 2 out of 5 votes, not a majority.
Figure 3 – If the file share witness was moved to the Alternate Site a failure of the WAN would cause a false failover as the Alternate Site would form a majority and come online.
Figure 3 – If the file share witness was moved to the Alternate Site a failure of the WAN would cause a false failover as the Alternate Site would form a majority and come online.
Figure 4 – with the file share witness in a 3rd location failover will occur if the Primary Site is lost and false failovers are avoided in the case of connectivity failure between the Primary and Alternate Site.
Figure 4 – with the file share witness in a 3rd location failover will occur if the Primary Site is lost and false failovers are avoided in the case of connectivity failure between the Primary and Alternate Site.

As you can see, figure 4 represents the only reasonable configuration which supports automatic failover. However, this assumes that there are an equal number of nodes in each location. If you are stuck with the original 3-node configuration you are stuck as adding a file share witness does not help as you can never achieve a majority in the alternate site…until today! Microsoft release a patch that basically allows you to specify whether or not a node gets to vote or not. So what this means is you can build a 3-node cluster as illustrated in Figure 1, yet take advantage a file share witness in a 3rd location as illustrated in Figure 4. By simply telling one of the nodes in the Primary Site to note vote in the cluster you will allow the Alternate Site to form a majority with the file share witness and come online. Assuming connectivity to your 3rd location and Alternate Site is relatively reliable there really is no downside to the configuration shown in Figure 5.

Figure 5 – by disabling the vote on Node2 you can deploy a 3-node multisite cluster with a file share witness and safely support automatic failover to the DR site. The same concept can be applied to any cluster with an odd number of nodes.
Figure 5 – by disabling the vote on Node2 you can deploy a 3-node multisite cluster with a file share witness and safely support automatic failover to the DR site. The same concept can be applied to any cluster with an odd number of nodes.

While this is a great solution, you still need that 3rd location for the file share witness. If you don’t have that 3rd location you will just have to settle for a manual switchover and keep the file share witness in the primary site if you have an even number of nodes.

The PreventQuorum switch is also included as part of this hotfix which will also be of interest to people deploying multisite clusters. Well explore that option in a future article.

Get the hot fix here…

A hotfix is available to let you configure a cluster node that does not have quorum votes in Windows Server 2008 and in Windows Server 2008 R2

http://support.microsoft.com/kb/2494036

Reproduced with permission from https://clusteringformeremortals.com/2011/04/29/microsoft-multisite-cluster18-users-rejoice-it-is-now-possible-to-have-automatic-failover-in-a-3-node-cluster19/

Configuring The Microsoft iSCSI Target Software For Use In A Cluster

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Now that Starwind has stopped offering a free, limited version of their iSCSI target software you might be looking for an alternative for your labs. If you need something for use in the cluster, check out Microsoft which has recently made their iSCSI target software available as part of the Windows Storage Server 2008 R2 download on Tech-Net and MSDN. It is not for use in production and has some of its own licensing restrictions, but it works fine and it is free for Tech-Net and MSDN subscribers.

Ready to start your shared storage cluster?

I recorded some really quick and dirty videos that aim to show you how to configure the iSCSI target and iSCSI initiator software in under 7 minutes. At the end, you will have a shared disk array ready to start your shared storage cluster. Hopefully when I get some more time I’ll actually write these steps out, but in a pinch this will give you the general idea of what needs to be done. There are plenty of other features, but for a lab environment this will do the trick.

http://screencast.com/t/2qUUDvZo6Zka – configuring the iSCSI target software and iSCSI initiator on the client

http://screencast.com/t/7m9ElSIdAbP – configuring the iSCSI initiator….continued

Reproduced with permission from https://clusteringformeremortals.com/2011/03/24/configuring-the-microsoft-iscsi-target-software-for-use-in-a-cluster/

 

You Might Want To Hold Off On The SP1 Upgrade For Your Cluster

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There are some reports on the newsgroups of storage validation failures after upgrading 3+ node clusters to Windows Server 2008 R2 SP1. It seems Microsoft is aware of the issue and they are investigating. I have been following this thread to keep up to date on the issue

http://social.technet.microsoft.com/Forums/en-US/winserverClustering/thread/e67026af-7f9b-4698-9ca7-11853431ed86

Intend to upgrade to 3+ node clusters?

You might want to wait for some sort of resolution before you apply SP1 to your clusters.

Reproduced with permission from https://clusteringformeremortals.com/2011/03/31/you-might-want-to-hold-off-on-the-sp1-upgrade-for-your-cluster/

SQL Server Denali And Hadron

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Microsoft’s latest version of SQL, code named Denali, is currently in CTP. One of the most anticipated feature is something called HADRON. Basically, it is has been described as mirroring 2.0. The best features of HADRON as I can see compared to mirroring as it is in 2005/2008 are as follows:

  • You can query the mirror (now known as a replica)
  • You can have more than one replica
  • Database are grouped into availability groups to ensure related databases always fail over together
  • Support for replicating more than 10 databases

If you want a great preview, check out fellow MVP Brent Ozar’s post, SQL Server Denali Rocks!

From reading Brent’s post I gather Microsoft only plans to include this in the Enterprise version of SQL. That is too bad for the many customers running SQL Server Standard as traditionally there is a significant price jump to go from Standard to Enterprise. However, that is good news for the 3rd party replication vendors out there who are providing similar functionality today on SQL 2005/2008 on Standard and Enterprise Editions.

SteelEye DataKeeper Cluster Edition for SQL Server

While it is not an exact duplicate of HADRON, a multi-site cluster using SteelEye DataKeeper Cluster Edition for SQL Server gives you a great solution for HA/DR without requiring shared storage. So while we wait for Denali and all the application vendors to start supporting Denali you might want to have a look at what SteelEye DataKeeper Cluster Edition can do for you today!

Reproduced with permission from https://clusteringformeremortals.com/2010/12/10/sql-server-denali-and-hadron/

Important Security Update For Windows Server 2008 R2 Clusters

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It seems as if there could be a vulnerability which would allow unauthorized access to administrative shares! You will certainly want this patch ASAP and also don’t forget to check the permission levels on your administrative shares after you apply this patch for Windows Server 2008 R2 Clusters.

http://www.microsoft.com/technet/security/bulletin/MS10-086.mspx

Recommendation. Microsoft recommends that customers consider applying the security update using update management software, or by checking for updates using the Microsoft Update service. Additionally, after applying the update, customers should check the permissions on existing cluster disk administrative shares and set the proper access levels for their environment.

 

Reproduced with permission from https://clusteringformeremortals.com/2010/10/13/important-security-update-for-windows-server-2008-r2-clusters/