I’m working on a disaster-recovery strategy at my day job, and AWS is one of the platforms I’m considering: the thinking is to have some capacity available on AWS, perhaps just a database and a couple of EC2 instances, ready to take some traffic if our main provider has serious and continued availability issues.
Cost is, as always, a big factor in the equation: we’re essentially building ourselves some insurance that we’ll stay up despite major problems, and like with all insurances we want to pay the smallest possible premium. Looking at the AWS pricing model, reserved instances immediately look like a way to both ensure capacity and get cheaper prices with respect to on-demand instances.
AWS reserved instances come in 1-year and 3-year types: the difference is in how much you pay upfront vs. how much you pay each month.
The question that comes to mind is:
should we buy 3-yr or 1-yr reserved instances?
To put it differently, what’s the chance that buying a 3-yr reserved instance today will give us a better deal, price-wise, than a 1-yr reserved instance?