This is the third article in our series comparing the AWS Reserved Instances and AWS Savings Plan models. This time we are going to take a closer look at the types of AWS Reserved Instances.

The discounts for AWS Reserved Instances apply in proportion to the time span that you commit to the usage of a particular instance type. The discount is applied to the on-demand rate on a per hour basis. Reserved Instances can be bought and used for resources such as Redshift, EC2, RDS, and ElastiCache. They come in two basic flavors: Standard Reserved Instances (RI) and Convertible Reserved Instances (RI).

A Standard Reserved Instance comes with more restrictions - it will allow you to change a limited number of attributes. For instance, a standard RI will not allow you to change the instance family, but will give you an option to change the instance size as part of that family. At the same time, you will be able to modify the availability zone of the regional Reserved Instances without being allowed to do the same for the zones.  

Compared to Standard Reserved Instances, Convertible Reserved Instances are less restrictive. They are more flexible - they will allow you to swap the existing RI with another convertible RI of a different configuration, which could include instance size and family, platform, payment option, tenancy, etc. AWS users can get a new RI with all of these under the condition that its value is the same as or greater than the original value of the RI, and that it is used in the same region.

As we said, the more limited an attribute is, the more attractive the discount will be. Convertible Reserved Instances offer lower discounts – 66% compared with 72% for Standard Reserved Instances, but that is the price tag that comes with the benefit of more flexible options.

In the next article, we are going to take a closer look at the types of AWS Savings Plans.