Reserved Instances are an enormous investment.
At first glance, that statement might seem counter-intuitive. Reserved Instances (RIs) are widely advertised as the best way to save big on your Amazon Web Service (AWS) cloud compute bill. And in many cases, they are. With Reserved Instances, companies commit to long-term usage by agreeing to rent virtual machines for a set amount of time (typically 1 to 3 years) in exchange for a significantly lower rate than on-demand pricing. When viewed through this lens, they appear to be a vital part of an AWS cost management strategy.
Cost Savings
Take Amazon EC2 as an example. When compared to on-demand pricing, Amazon EC2 RIs offer customers potentially deep discounts — sometimes as much as 75%, per their marketing. While reserving cloud capacity in advance seems like the smart thing to do because it has the potential to deliver a significant amount of savings, the savings promised by RIs often have a dangerous downside — and any missteps can have substantial costs for your company.
The calculations involved in deciding which RI to purchase can be frustratingly complicated. One year or 3 year contract? What about tenancy? Instance size? Region and zone? New or from the marketplace? And don’t forget about the nuance of offering class — do you want your RI standard, convertible or scheduled?
These calculations are difficult, but absolutely vital when committing to a RI. Rather than signing a contract for exactly what you have now (in terms of size, region, and tenancy) and guessing at the length of time that will fit your instance, it’s essential to understand the exact shape of your usage needs. Without that kind of granular insight into your workload, it’s impossible to choose a RI that will be the right fit six months from now, let alone three years in the future.
In the end, many companies buy RI capacity that ends up exceeding their actual needs, because they’re already using capacity that exceeds their needs. Unfortunately, committing to more capacity than you actually need can be very costly over the length of a RI contract. When that happens, the long-term return on investment (ROI) ultimately evaporates.
Read More “Before You Buy a Reserved Instance, Read This”