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When buying reserved instances on AWS, you can choose what size of instance you want to buy.

But:

My question is, why would you ever buy anything but the smallest size?

For example, if you run a small instance in EC2, you might be tempted to buy a small reserved instance (3 years, all up front, $229). small instances have a normalization factor of 1. Why not buy four nano instances instead, which have a normalization factor of 0.25, and cost $228 (a buck cheaper)?

Wouldn't it always be better to purchase the smaller ones so you have the greatest flexibility? Why does Amazon even sell the larger sizes?

mlissner
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  • Thanks for the downvote. I wish I knew what was wrong with the question. – mlissner Jul 08 '22 at 20:56
  • I assume that even if people are aware of this possibility they would still simply buy what they (think) they need and won't look for a route to save a single $1 (on their corporate credit card) – Rob Jul 09 '22 at 07:39
  • I think you’ve missed the most basic factor in purchasing reserved instances. You HAVE to purchase reserved instances that match the existing type of running EC2 instance. If you have an RI type that matches a running EC2 type then that EC2 instance is billed at the reduced price of the RI. You can not purchase a smaller RI instance and have it apply to a larger EC2 instance. That’s part of the problem with RIs. You are committing to use a specific size EC2 instance for a reduced price over a longer period. – Appleoddity Jul 09 '22 at 16:08
  • No, not really. "A Reserved Instance that is purchased for a Region is called a regional Reserved Instance, and provides Availability Zone and instance size flexibility." From [here](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/apply_ri.html). – mlissner Jul 10 '22 at 11:12
  • @Rob, it's not about the dollar savings. It's about the flexibility. Say you buy a there year xlarge reservation to match your xlarge instance, then a week later need to downsize to a large instance. Now, and the end of your three years, you have extra RIs you haven't used and can't sell. Now imagine you bought a bunch of micro RIs for the xlarge instead and you downsize again to large. In this case, you can sell the extra micro RIs on the marketplace. Better! I think nobody here understands this. – mlissner Jul 10 '22 at 11:20

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I'm feeling vindicated on this one. The AWS Cost Management Reservation Recommendation tool now recommends lots of micro reservations when you have larger databases.

For example, we have zero micro instances, and have a bunch of xlarge instances. Instead of recommending that we buy xlarge instances, AWS recommends we buy 56 micro instances, and even explains:

To save an estimated $2,894.08 annually, we recommend you purchase db.t3.micro Reserved Instances to cover 28 normalized units per hour of db.t3 family usage to maximize savings over a 1-year term.

Screenshot telling us to buy 56 micro instance

So if you want the greatest flexibility, AWS recommends it, and now I do too. I haven't heard any explanation why you'd buy anything but the smallest instances in the family you need.

mlissner
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