After you purchase a Reserved Instance (RI), RI will be automatically matched with a pay-as-you-go instance within the validity period. Currently, RI only supports matching pay-as-you-go instances running on Linux. If there is no pay-as-you-go instance available for matching under your account, RI will be idle, but fees still incur. Eligible pay-as-you-go instances will be automatically matched immediately after purchase, and once the match is successful, the discount will take effect on the bill for the pay-as-you-go instance.
For example, if you purchase an RI of model S3.16xlarge256 in Silicon Valley Zone 1, and three pay-as-you-go instances of model S3.16xlarge256 with the same attributes in the same availability zone are running for one hour each under the current account, then only one pay-as-you-go instance is billed for one hour at the RI price, and the other two instances are billed for 2 hours at the pay-as-you-go price.
However, if you purchase an RI of model S3.16xlarge256 in Silicon Valley Zone 1, and run three pay-as-you-go instances (A, B, and C) with the same attributes in the same availability zone for 20 minutes each, then the total running time of the instances is one hour, and one hour of RI usage and 0 hours of pay-as-you-go usage is incurred. As shown in the figure below, those three instances are matched with the RI for 20 minutes each.
If three matching pay-as-you-go instances are running at the same time, the RI billing advantage is applicable to all the instances in one hour (up to 3,600 seconds); after that duration, they will be charged at the pay-as-you-go price.
The paid RI cycle, if not created on the hour, takes effect on the last hour prior to its creation and runs a full year span of 365 days. The RIs created on the hour fall under the typical billing cycle of one year and stay effective till the same hour next year. Take the following two examples.
If you purchase an RI at 13:25, the discount will be applicable from 13:00 to 14:00 on the same day next year.
If you purchase an RI at 13:00, the RI discount takes effect at 13:00 and will be valid for the next 365 days.
For more Billing Rules information, please refer to Reserved Instance Billing Mode.