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We are setting up a new Disaster Recovery environment. On Production we have SQL Server cluster with two nodes. For DR we decided that one node should be enough. The plan is to add the second node ASAP if disaster happens.

We requested our managed cloud company to create a single node cluster for us. However, they try to discourage us from doing that and suggest installing non-clustered instance instead, so when disaster happens we will build brand a brand new cluster with two brand new nodes and then migrate to it from non-clustered instance. This option sounds like more work for us if disaster happens. We will need to migrate all the databases to the clustered instance from non-clustered instance. I also expect a longer downtime if we do that, as the opposite to just adding a second node to the existing cluster.

Tech person from this cloud company said that single node cluster is complicated solution and is prone to all kinds of problems like "split brain", that it can stuck all of a sudden and they have seen so many issues with single cluster nodes. He said that Microsoft does not recommend having single node clusters. He wasn't very specific and I'm not sure I understood what exactly problems we will have.

I tried to google for some specific problems, but failed to find were Microsoft would suggest to avoid having single node clusters, especially when it comes to DR.

Is there a reason to avoid single node cluster solution for DR and go with non-clustered instance instead?

Alex Aza
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  • Why don't you add the third node to your existing cluster? – joeqwerty Mar 07 '15 at 18:29
  • While I've never set up a single node cluster, I can say that split-brain is impossible when you have a single node. As I understand it split brain is when two nodes think they are the only active node, causing file corruption and other fun things. – Reaces Mar 07 '15 at 21:32
  • Why are you using a cluster when always on would do all of the items you ask? You only need to configure a very basic cluster for it to work no clustered disks required. –  Mar 08 '15 at 04:58
  • @joeqwerty - adding a third node to existing cluster will not help DR environment for many reasons. The main goal here is to have a separate copy of everything we have on production in another geographically isolated environment. Should natural disaster happen and destroy our production data center completely there will be fully functional DR environment that will become our production then. All cluster nodes work with the same SAN. Adding another node does not add any redundancy to the SAN. – Alex Aza Mar 08 '15 at 06:13
  • @Adam - Failover Cluster is part of AlwaysOn. If you ask why we do not use AlwaysOn AG instead, it would be whole new topic and I would rather not discuss. Every HA technology has advantages and disadvantages. There are always reasons to choose one over another depending on your needs, goals, SLAs, etc. – Alex Aza Mar 08 '15 at 06:29

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