Purchase reserved instances for Aurora with multiple read-write instances and multi-AZ

Question

We have an on-demand Aurora DB composed of 
- a read/write instance, and
- a read-only replica instance,
- all duplicated in a second AZ, for a total of 4 instances.

We want to purchase a reserved instances plan for all, but on the form when I select "aurora-mysql", in the "Deployment Option" it disables "Multi-AZ DB Instance" and autoselects "Single-AZ DB Instance".

So... after reading tutorials and watching videos I'm still not completely sure about what value to input.

Must I execute 2 different purchases with "2" in "Number of DB instances", or a single purchase with "4" instances a single time?

Answer

Please refere to our document for more detail: https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/USER_WorkingWithReservedDBInstances.html
```
Note
Reserved Amazon Aurora instances always have the deployment option set to Single-AZ DB instance. However, when you create an Aurora DB cluster, the default deployment option is Create an Aurora Replica or Reader in a different AZ (Multi-AZ).
You must purchase a reserved DB instance for each DB instance you plan to use, including Aurora Replicas. Therefore, for Multi-AZ deployments on Aurora, you must purchase extra reserved DB instances.
```

Example:

Aurora cluster consists below instances:
- db.r6g.2xlarge: Writer
- db.r6g.2xlarge * 3:  Reader

In this case, you should purchase `db.r6g.2xlarge` RI `4 instances`.

If you need billing specific support, please open a support case to customer support to confirm see if you can choose right purchase option.

Purchase reserved instances for Aurora with multiple read-write instances and multi-AZ

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