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I have an RDS instance running of type db.t2.small. I bought a reserved instance for 1 year of the same type, I paid the partial upfront rate.

Do I need to assign this subscription in some way to my existing instance to say 'This is the instance that is using the subscription' or does AWS just charge for the instance usage depending which instances you have running and whether an available subscription was active for it at the time which determines the running rate?

The reason that I ask is that I can't see any mention of the reduced rate on my AWS Billing area and it shows that I've used 17 hours of my reservation, but there isn't a row on my bill so far this month that indicates the new partial upfront rate that I should be charged for my instance.

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No, you don't allocate an instance to the reservation. Read this, and this FAQ. I knew this already, but I found the information in 30 seconds on Google.

From the FAQ

Q: How do I purchase and start up a Reserved Instance?

Reserved Instances are not physical instances, so they don't have to be launched or started up. The reservation is automatically applied to running instances in your account, and you immediately start benefitting from the lower hourly cost. If you don't have a running instance that matches your reservation's specifications, you can launch one via the Amazon EC2 Console or call the RunInstances API method. AWS will automatically apply the cheapest, eligible rate.

Q: How do I control which instances are billed at the lower rate?

The RunInstances API command does not distinguish between On-Demand instances and the reservations that can be applied to them. When computing your bill, our system will automatically optimize which instances are charged at the lower rate to ensure you always pay the lowest amount.

  • I spent a good while searching for the answer to this, thanks for pointing it out in the FAQs. I'm surprised it isn't mentioned in the AWS documentation for reserved instances. I literally couldn't see it anywhere. – Luke Feb 27 '16 at 16:07

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