Why Can't I Select a Multi-AZ DB Instance When Purchasing Reserved Instances?

When purchasing Reserved Instances for your Aurora PostgreSQL cluster with Multi-AZ configuration, you're correct in observing that you can't directly select Multi-AZ DB instances. However, this doesn't mean you can't benefit from Reserved Instances for your Multi-AZ setup.

For Multi-AZ DB clusters, including Aurora PostgreSQL, you can purchase Reserved Instances for Single-AZ DB instances, and these will apply to your Multi-AZ setup. The key is to match the instance type and size of your Multi-AZ cluster's DB instances.

Here's how it works:

1. For a typical Multi-AZ DB cluster with three DB instances (one writer and two readers), you would purchase three Single-AZ Reserved Instances of the same instance type and size as your cluster's DB instances.

2. The Reserved Instance discount will automatically apply to your Multi-AZ cluster's DB instances, as long as the instance types match.

3. If you purchase fewer Reserved Instances than the total number of DB instances in your cluster, your cluster will be partially reserved. For example, if you have a cluster with three db.m6gd.large instances and you purchase only one db.m6gd.large Single-AZ Reserved Instance, only one of your three instances will be covered by the reservation. The remaining two will be charged at the on-demand rate.

4. Size-flexible Reserved Instances are available for certain database engines, which allows you to apply your reservation to larger capacity DB instances within the same instance class type and AWS Region.

The reason Multi-AZ options are not directly available for Reserved Instances is that from a billing perspective, Multi-AZ deployments are treated as multiple Single-AZ instances. This approach provides more flexibility in how you can apply your reservations.

To ensure your Multi-AZ Aurora PostgreSQL cluster benefits from Reserved Instances:

1. Determine the instance type and size of your DB instances in the cluster.
2. Purchase the same number of Single-AZ Reserved Instances as you have DB instances in your cluster, matching the instance type and size.
3. The discount will automatically apply to your Multi-AZ cluster.

By following this approach, you can still take advantage of the cost savings offered by Reserved Instances for your Multi-AZ Aurora PostgreSQL cluster.
Sources
Reserved DB instances for Amazon RDS - Amazon Relational Database Service