What does the RI waterline describe?

• A. The percentage of time the RI must be used to break even on upfront costs.
• B. The maximum number of instances a single RI can cover.
• C. The amount of usage from matching instances that yields savings, with no explicit threshold.
• D. The threshold usage percentage at which savings are realized (for example, using the RI more than 75% when it saves 25%).

Answer: (D)

The RI waterline describes the usage threshold at which a Reserved Instance starts delivering savings compared with on-demand pricing. It’s the percentage of your RI-covered usage that must be realized for the upfront or commitment cost to be justified. It’s not about how many instances you can cover, nor a fixed break-even time; it’s the tipping point where the accumulated savings from using the RI exceed the pre-paid cost. If your actual usage stays above this threshold, you realize savings; if it stays below, the RI may not be cost-effective. For example, if the waterline is around 75%, using the RI more than 75% of the time can yield the expected savings (such as saving 25%). The waterline can vary based on the specific RI terms, prices, region, and instance type, so you tailor it to your forecasted utilization.