Reserved Instances allow AWS customers to receive a significant discount on their EC2 usage (up to 75% when compared to On-Demand pricing), along with capacity reservation when the RIs are purchased for use in a specific Availability Zone (AZ).
Late last year we made Reserved Instances more flexible with the launch of Regional RIs that apply the discount to any AZ in the Region, along with Convertible RIs that allow you to change the instance family and other parameters associated with a Reserved Instance. Both types of RIs reduce your management overhead and provide you with additional options. When you use Regional RIs you can launch an instance without having to worry about launching in the AZ that is eligible for the RI discount. When you use Convertible RIs you can ensure that your RIs remain well-fitted to your usage, even as your choice of instance types and sizes varies over time.
Instance Size Flexibility
Effective March 1, your existing Regional RIs are even more flexible! All Regional Linux/UNIX RIs with shared tenancy now apply to all sizes of instances within an instance family and AWS region, even if you are using them across multiple accounts via Consolidated Billing. This will further reduce the time that you spend managing your RIs and will let you be even more creative and innovative with your use of compute resources.
All new and existing RIs are sized according to a normalization factor that is based on the instance size:
Let’s say you already own an RI for a c4.8xlarge. This RI now applies to any usage of a Linux/UNIX C4 instance with shared tenancy in the region. This could be:
- One c4.8xlarge instance.
- Two c4.4xlarge instances.
- Four c4.2xlarge instances.
- Sixteen c4.large instances.
It also includes other combinations such as one c4.4xlarge and eight c4.large instances.
If you own an RI that is smaller than the instance that you are running, you will be charged the pro-rated, On-Demand price for the excess. This means that you could buy an RI for a c4.4xlarge, use that instance most of the time, but scale up to a c4.8xlarge instance on occasion. We’ll do the math and you’ll pay only half of the On-Demand, per-hour price for the larger instance (as always, our goal is to give you access to compute power at the lowest possible cost). If you own an RI for a large instance and run a smaller instance, the RI price will apply to the smaller instance. However, the unused portion of the reservation will not accumulate.
This new-found flexibility is available now and will be applied automatically to your Regional Linux/UNIX RIs with shared tenancy, without any effort on your part.
Source: AWS Blog New – Instance Size Flexibility for EC2 Reserved Instances