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I am exploring FCI and AG.

In FCI we can have 2 nodes that share the same SAN storage. And failover results in the other node starting up.

Where as AG is for a set of dbs.

This question is about the following quote from msdn: "An FCI can participate in an availability group as one of the instances hosting either a primary or secondary replica.".

This sentence is confusing because An FCI can participate in an availability group as one of the instances means that only one side of the AG can be a FCI, which means the other side must be a stand-alone instance.

Reading the sentence fully An FCI can participate in an availability group as one of the instances hosting either a primary or secondary replica. - I think they are saying that - One or more AG participants can be a FCI, and that it can either host the primary or secondary. This makes more sense. Have I understood correctly?

During configuration of AG, assuming I have FCI on one or both sides, then should I add FCI to AG or should I add a single instance from the FCI into AG? I think the answer is FCI because when we can an instance is a FCI it means it is not a single instance. Correct?

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If you want to mix FCI and AG, you can do so and it is supported. Below is a hypothetical configuration.

Failover Cluster Instance (Node A, Node B), addressed as Server 1. All clients connect to Server 1. Only one Node is live at a time (i.e. only one node has control of the storage that has the databases on it.

Node C lives in your Disaster Recovery center. You don't want to go through the expense of making your SAN geographically dispersed so you can't just add it to the Failover Cluster Instance. So you setup an Availability Group. That availability group is on Node C <--> Server 1 (A/B).

Given my setup and my businesses existing tolerances for data loss, I would setup the AG for asynchronous commit and manual failover.

Direct Answers to questions

1 - The FCI can be either the primary or the secondary member of an availability group. To the availability group, it's just a server. Point the AG at the FCI cluster name though.

2 - Both nodes will participate (one at a time) in the AG. FCI is like two brains sharing a body. Only one can control the body at a time, so for all intents and purposes only one brain exists at a time. An AG is two complete setups, one shadowing another ready to step in at any time.

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  • 2 brains sharing body. So I must give the fci cluster name and never worry about whether it is fci node 1 or fci node 2 participating in the AG right?
    – variable
    Oct 8, 2021 at 13:13
  • The FCI can be either the primary or the secondary member of an availability group. - Can FCI be on both the sides (Primary site and DR site)?
    – variable
    Jul 19 at 9:58

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