[originally posted on LinkedIn Pulse @ http://linkd.in/1HndUik]
You can take your reservation discount with you, *almost* anywhere you go, provided you have the insight to do so. This ability is perhaps one of the best benefits of Amazon’s Reserved Instances as it provides insurance for those hesitant to commit to 1 or 3 year reservation terms.
This way, when your usage needs change, you simply switch your reservations to carry your cost reduction and capacity reservation wherever your usage leads you.
Let’s imagine you’ve prepaid 2 Linux m3.medium reservations in us-east-1d but your usage has shifted almost entirely to Linux m3.large instances in us-east-1e. You can make an equal exchange of 2 m3.medium instance reservations for 1 m3.large (equal in the sense that the footprint is the same) and swap the AZ to match your current On-Demand usage. This will help to close the gap between number of reservations available versus number of instances in use.
By doing so, you ensure that that you receive the On-Demand pricing percentage discount you’re entitled to. Otherwise, you’re missing out on the significant hourly discount and not chipping away at the amortized upfront amount you originally made to Amazon for the reservation.
While there are some cases where reservations cannot be modified (e.g.if terminating hours do not match as well as licensing restrictions on Windows boxes, etc.), the majority of them can easily be modified in the following ways:
- Availability Zones within the same region (us-east-1a –> us-east-1c)
- EC2-VPC and EC2-Classic
- Different instance type within the same instance family (2 m3.med for 1 m3.large)
As your needs change, you needn’t be held prisoner to your reservations, or write them off at a loss; however, RI modification does need to be an integral part of your RI management processes in order to maximize your return on investment. If not, you’re just leaving money on the cloud table.
Modifications can be submitted through the AWS console, the API or automatically via a cloud management platform like CloudHealth®. How often do you check for modifications?