Reservations, like savings plans, are a billing abstraction. In this case, you are committing to pay for a specific instance size, in a specific instance family, in an AZ or Region, in exchange for a discount. You do not have any assurance that you will actually be able to spin up that kind of instance however. Reservations also have some amount of flexibility, but upwards. For example, if you purchase 2 m3.large reservations, they will primarily apply to 2 running m3.large instances. If none are running, it will try to apply against a running m3.xlarge instance. Reservations are more limiting than savings plans and I would personally preference savings plans for their flexibility. More on reddit.com

I've got 3 more months on my t2.medium reservation and would love to switch to t4g for the next 3 years. Any chance this will happen soon u/jeffbarr ? More on reddit.com

We run quarterly assessments to figure out how many instances are running 24x7, and then further break that down to understand the long term strategy for those particular systems / platforms. If we expect it to be on more than 7 months, we buy an RI for it. We have a lot of traditional on premises platforms that have mostly lifted and shifted into the cloud, so RIs make a ton of sense for us. Likewise, even more elastic and scaling platforms still have minimum baseline compute running at all times and use auto scaling to scale up during peak periods. It still makes sense to buy RIs for those systems, as there’s always X running 24x7. Managing them isn’t too hard. Once you buy them, it’s set and forget until the next quarterly review. RIs don’t tie to a specific instance, so even if we decommission a platform earlier than expected, we can shut those instances down and use the RIs on other systems that were more recently spun up and not covered. We use cloudhealth to do this mostly automatic for us. They provide RI recommendations, we look them over and act accordingly. We have roughly $2m a year in spend and RIs helped us cut that down to about $1.2m effectively overnight. We do 1 year no up front exclusively. More on reddit.com

Convertible RIs end up being the same price as Compute Savings Plans, and standard RIs end up being the same price as EC2 Savings Plans. For me, Savings Plans have completely rendedered RIs obsolete, especially the CSP (as opposed to the EC2SP). No instance type or region restrictions -- just spin up your instances, change them, do whatever, and it will be billed at the lower rate. No need to "trade in" convertible instances, etc. For your use case where you know you will be within a single instance family in a single region, you may as well use the EC2SP. You just commit to $x/hr and it doesn't matter if you spin up 16 t3.small or 8 t3.medium or 1 t3.2xlarge... it will just be billed at the lower rate and you can change the instance type at will. More on reddit.com