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Instance Size Flexibility With Reserved Instances

Yesterday, AWS announced that their regional benefit EC2 reserved instances can be applied against any EC2 instance in the same EC2 instance family. We’ll explain to you exactly what that means.

Today, under the standard reserved instances, default tenancy umbrella, we have 2 main types of reserved instances:

  • Those that reserve capacity and provide a cost benefit
  • Those that only provide a cost benefit

Reserved instances that are tied to a specific availability zone (AZ), will reserve capacity as well as provide a cost benefit. With this type of reserved instance, AWS is promising to you that if you need that type of EC2 instance in that AZ, then it will have it waiting for you.

These AZ-specific reserved instances are not covered by yesterday’s announcement.

Alternatively, there are regional benefit EC2 reserved instances. Regional benefit reserved instances were announced back in September. With these reserved instances, you can get a discount on the cost of your EC2 instance, but there is no capacity commitment from AWS. So, just like a regular On Demand EC2 instance, if an availability zone is out of c4.large instances, and you need one, then you are out of luck. You’ll have to wait for one to be freed up by someone else.

For many people, the capacity limitation does not matter. It’s the price break that they are really after.

Regional benefit EC2 reserved instances is where yesterday’s announcement for instance size flexibility now applies.

Instance Size Flexibility

With yesterday’s announcement, your non-AZ-specific reserved instances can now apply to any EC2 instance size that’s in the same EC2 instance family. So if you have a reserved instance for an m4.xlarge, it can apply to an m4.xlarge instance, or 2 m4.large instances, or even half an m4.2xlarge instance. But it will not apply to any EC2 instances in other families, such as T2, M3, C3, C4, etc.

Stepping back a bit, when AWS introduced Convertible Reserved Instances, it created a normalization table where the instance sizes related to each other. In that table, you’ll notice that as you go down the table, the numbers double at each step. A medium is double a small. A large is double a medium. A 2xlarge is double a large, etc. You will also see this in the On Demand pricing for EC2 instances.

Mathematically, there are benefits to doing this: it takes the pricing out of the equation when determining instance size. You can decide if your infrastructure needs 2 large instances, or 4 medium instances without having to worry how price may affect things. Because the net result is that the price between those two will be identical.

AWS has now applied this same logic to their regional reserved instances. Now, if you have a reserved instance, you don’t need to worry about accidentally losing it if your infrastructure needs change.

For example, if you’ve been using your m4.large reserved instance for 6 months, and now you need an m4.xlarge instance, you can make the change and AWS will still apply the reserved instance against your instance. In this case, it will apply it for 30 minutes of each hour. Standard On Demand pricing will apply for the other 30 minutes.

Going a bit further, if you have 2 m4.large reserved instances, and an m4.xlarge EC2 instance running, AWS will now apply both reserved instances against your single EC2 instance. Each will apply for 30 minutes of each hour.

Important Restrictions

The new instance size flexibility applies only to EC2 instances and reserved instances within the same EC2 instance family (T2, M3, M4, C3, C4, etc.)

This announcement only applies to standard reserved instances using default tenancy. So convertible reserved instances are not included.

Currently, this only applies to Linux instances at the moment. Windows users cannot benefit from this. Hopefully that will change soon.

Combining With Scheduled Right-Sizing

This has tremendous repercussions.

In the past, there was a decision to be made: right-size or buy a reserved instance. Now, you can combine reserved instances with scheduled right-sizing for even more savings.

For example, you can purchase a regional benefit reserved instance for an m4.large. Then during business hours, you can scale up your EC2 instance to an m4.xlarge when the load is required and the reserved instance will still apply.

If you use Skeddly, scheduling right-sizing of your EC2 instance is easy.

Final Thoughts

Flexible instance sizes in regional reserved instances is simply a net win for everyone. And the best part is that it gets applied automatically. Unlike convertible reserved instances, you don’t have to do anything to the reserved instance.

About Skeddly

Skeddly is the leading scheduling service for your AWS account. Using Skeddly, you can:

  • Right-size your EC2 instances to reduce costs while optimizing availability,
  • Start and stop your EC2 instances to eliminate costs when your instances are not needed.

Sign-up for our 30 day free trial or sign-in to your Skeddly account to get started.