Sql-server – Patching strategy when Windows and SQL Server patches not applied for over 3 years

You want to choose "Add node to a SQL Server failover cluster". When creating a Failover Cluster Instance (FCI), you will always do one initial cluster installation for the instance, then add nodes to that installation.

Keep in mind that Active/Active and Active/Passive as descriptors is very misleading. By it's very nature, each SQL Instance you install can only be active on a single node at any one time. To leverage multiple physical nodes at the same time, you will need to install multiple named FCIs to your cluster.

For example, let's assume a two node cluster. If we install a single named FCI to this, our layout would look like this:

NODE1:  CLSTSQL01\SQLServer01 (Active)
NODE2: 

This would be the result of you performing one "New SQL Server Failover Cluster installation" with the additional node additions for each node in your cluster. Now let's add a second instance to it so we can utilize each node. For this, we would install a second FCI to the cluster and end up with something like this:

NODE1:  CLSTSQL01\SQLServer01 (Active)
NODE2:  CLSTSQL02\SQLServer02 (Active)

Now, keep in mind that in a failover situation, on either node, the SQL instances could end up on the same node. For example, if NODE1 failed:

NODE1: <<DEAD>>
NODE2: CLSTSQL01\SQLServer01, CLSTSQL02\SQLServer02 (Active)

This is the basics of how you would arrange your instances. You will need to monitor your cluster if you want to make sure your instances are properly balanced across your nodes.

Elijah. There's two separate questions here:

1. Is DTC supported with AlwaysOn Availability Groups?

You're using SQL Server 2012, and according to Microsoft's Documentation, that answer is no. I totally understand that you want to try it anyway, but keep in mind that you're now putting something into production that Microsoft simply will not support, AND you're using two separate niche features together (AGs and DTC). If anything whatsoever goes wrong, you're going to be in a world of hurt. This just isn't something I'd ever even think about trying in production.

Keep in mind that if your managers find out that you deployed something Microsoft specifically says in big letters, "YOU CAN'T DO THIS," and you have any kind of outage where you have to call Microsoft for support, you're going to have some ugly explaining to do.

Technically, DTC is supported starting with SQL Server 2016 SP2 and later, but it just means that you can pick which database loses data on failover, and the application has no idea data was permanently lost. That's not what a normal database administrator would call DTC support.

2. How should DTC be configured in a multi-node, multi-subnet cluster?

Read Allan Hirt's post on configuring DTC with multiple instances of SQL Server in a cluster, and make sure to read all of the links in the post as well.