During one of my recent classes on FinOps, I was asked if I could describe some of the pros and cons of convertible vs standard Amazon Web Services Reserved Instances (AWS RI). Specifically, why a user shouldn't just buy convertible RI rather than purchasing standard RI. We had a great back and forth on this subject, and a few points stood out that I thought would be worth sharing.
Flexibility or Savings?
Before you decide which type of RI that you'll purchase consider what your end goal is, flexibility or savings. If you answered that question with savings, then let's start with a standard RI. A standard AWS RI will provide the highest discounted rate on a single instance type. This is a great option if you're looking for a discounted rate on an instance type that has high usage, is rarely changed, and will continue to be used for a committed time. For instance, if you have a host of t3 instances running 100% of the time there's no reason to look at flexibility, instead, just concentrate on your savings with a standard RI purchase.
If you are anticipating some flexibility in your cloud architecture, then you'll want to consider utilizing convertible RI. A convertible RI can be exchanged for different configurations, including changing the instance families, operating systems, or tenancy. There are no limits on how many times you make these changes, as long as the new version of the convertible RI is of an equal or higher value than the original that you're exchanging. There may be "true-up" costs involved with the exchanges, but these really can shine in highly flexible environments.
Modifying RI or Convertible RI
The second question that was raised was why you couldn't just modify your RI like you would the convertible RI. The understanding was that it's possible to modify existing AWS RI so it wasn't as necessary to purchase convertible RI. Unfortunately, that's not a complete understanding of the limitations of a modified RI. A convertible RI can be exchanged in several different ways that a standard RI can't be modified into. Here are a few of the top-level examples of the differences:
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Modify RI
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Convertible RI
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Change the Scope from Availability Zone to Region (or vice versa).
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Convertible RI is associated with a specific region and cannot be changed between regions
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Change the network from EC2-Classic to Amazon VPC and vice versa
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Convertible RI can only be exchanged for another convertible RI.
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Change the instance size within the same family
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Can be changed between instance types and instance sizes
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As you can see once again, we're looking at the type of flexibility that's needed. If your organization is changing its overall cloud infrastructure to new instance types you'll want the flexibility of a Convertible RI. If you are only modifying the instance sizes you're using, not the instance type, you may want to only stay with standard RI for greater savings instead of worrying about flexibility.
The points I've mentioned above are not all of the considerations that you'll need to keep in mind as you decide which of the RI you'll purchase, but they should give you a great starting point. I also encourage you to read through the AWS documentation:
After you get comfortable with the two types of RI make sure to take the time to talk with your Cloud Engineers. Consider the need for either flexibility or cost savings, discuss the pros and cons of each type of reservation, and make sure you feel comfortable with changes that may be coming to your cloud environments..
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Justin Kean
Apptio
Sr Instructor
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#Cloudability