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  • AO: Always ON
  • FCI: Failover Cluster Instance
  • WSFC: Windows Server Failover Cluster

I'm scratching the surface of AO. I currently have one SQL 2012 FCI on a WSFC. Which means one Active node at a given time. I cannot set up an AO with this, can I? Logically speaking I need “at least two” active (up & running) SQL Server instances to set up. And in Microsoft's example they used a total of 5 nodes, and the minimum would be 3 for a AO on FCI + WSFC right?

Here is what my FCI looks like.

There is no point in having AO when only 1 instance of SQL is running at a given time. Right ?

If yes what would I need to set up AO?

  • A new Node in the Cluster or
  • Any other Stand alone SQL Should(same version) do as well?
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  • AlwaysOn is a marketing term and describes a whole bunch of things - failover clustering, mirroring, Availability Groups, even log shipping. When you say "Always ON" do you mean "Availability Groups"?
    – Aaron Bertrand
    Apr 25, 2014 at 16:04

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If by AlwaysOn you mean "Availability Groups" (which is typically the case), then you can most definitely use your failover cluster instance as a replica.

For this to be an Availability Groups design that is actually useful in the sense of AGs, then you would most likely want another server to host another replica in the Availability Group (for whatever it is that you're looking for: HA, DR, reporting, etc.). All of the servers that are going to be participating in the Availability Group need to reside in the same WSFC.

There is no point in having AO when only 1 instance of SQL is running at a given time

The replicas in an Availability Group are individual SQL Server instances (whether it be a FCI or standalone instance), so each of the replicas are "running" in a normal condition.

what would I need to set up AO?

Technically speaking? You would just need another instance of SQL Server to have as an additional replica for the AG.

Realistically speaking? We can't answer that right now with the current state of the question. We'd need to know exactly what you want your Availability Group for.

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  • Thank you, makes sense. Yes I meant Availability groups. I need AG for high availability for a group of datbases. Can I do that with in the scope of an Active passive SQL cluster for offloading reporting work on certain databases.
    – user3154761
    Apr 25, 2014 at 16:40
  • You won't be able to have automatic failover with an FCI as the primary. The FCI would be your unit of auto failover and unattended HA. Does that answer your question?
    – Thomas Stringer
    Apr 25, 2014 at 17:51
  • Also there is no such thing as an "Active passive SQL cluster" - you can't offload reporting to the "passive" node because the passive node sits there without an instance of SQL Server running. It's there to serve as a host for the instance if the primary server goes down.
    – Aaron Bertrand
    Apr 25, 2014 at 18:25

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