Back in March, when Amazon first came out with reserved instances for EC2 (instances you can pre-pay at a discount for a period of 1 or 3 years) I posted Amazon Reserved Instances: Do They Make Business Sense? In that post I also made available a public Zoho spreadsheet with a simple calculator that allows figuring out whether reserved instances make sense for you or not.
I also created a version for the European prices (full, editable version here):
The break-even point for 1-year instances is 3,250 hours, or about 4.5 months (assuming you are running the instances 24/7). The break-even point for 3-year reserved instances is 5,000 hours, or about 7 months (assuming you are running the instances 24/7).
Several of the companies I have been working with have a substantial pool of instances constantly running. And although they are accounting for reserved instances in their business plans to demonstrate the long-term benefits of the gross margins they allow, they seem hesitant to actually pay for reserved instances. These calculations demonstrate that they are probably being too risk averse as in their case, the platforms/apps are sure to be running for at least 7 months and there are substantial cost-savings involved.
Are you considering or already running reserved instances? What is the use case and what are the pros and cons? Please share in the comments.