RDS RI Deployment type

Question

A customer is interested in RDS RIs, but confused with our description of what is fixed for RIs.

According to [this page][1]
On one hand "Region, DB Engine, DB Instance Class, Deployment Type and term length must be chosen at purchase, and cannot be changed later.". 
As I understand, deployment type is Single or Multi-AZ, so it can't be changed after the purchase.

But on the other hand, "The RI discounted rate will also apply to usage to both Single-AZ and Multi-AZ configurations for the same database engine and instance family. For example, let’s say you purchased a db.r3.large PostgreSQL Single-AZ RI in EU (Frankfurt). The discounted rate of this RI can automatically apply to 50% of the usage of a db.r3.large PostgreSQL Multi-AZ instance in the same region."

Does it mean that I can change deployment type (Single -> Multi) for the reserved instance, but in this case the only 50% of price will be covered by RI purchase?

[1]: https://aws.amazon.com/rds/reserved-instances/

Accepted Answer

What you may be confused about is size flexibility. In 2017 RDS implemented size flexibility for certain RDS platforms: https://aws.amazon.com/about-aws/whats-new/2017/10/amazon-rds-reserved-instances-offer-instance-size-flexibility/

Non-licensed RDS platforms can be size flexible. The RI purchased with the size flexible platform type, can be applied to db instances regardless of their deployment type (single- or multi-AZ) and regardless of the instance size within the same family AND generation.

However, you CANNOT modify or change any of the attributes of RDS RI itself, after you purchase it. You have to select the attributes at the beginning, when you purchase it.

Because of the size flexibility, it makes sence to just purchase RIs with the lowest instance size within the family and single-AZ deployment type. You only need to calculate how many of them you need based on the normalization factor (same normalization factors apply as with EC2): scroll to "Instance size footprint" in the docs here https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ri-modifying.html

Above link is about EC2 RI modification, but it has a table with the normalization factors, which can be used for RDS db instances footprint calculation as well.

RDS RI Deployment type

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