Hi,

We've been using AWS since 2012 and we've never actually switched to a reserved instance because we don't really understand how it all works...

We have:

• 1x t2.micro

• 1x t2.small

• 1x t3.large

Our t2.micro instance changes now and again to t2.small and occasionally a t2.large, we probably make a change every 2~ months

Our t2.small rarely changes but every 6~ months or so it may get an increase or a decrease.

Our t3.large sometimes goes up to an t3.xlarge or even sometimes a c2.xlarge

We don't plan on leaving AWS anytime soon, we'll likely stay with aws for at least the next 3-5 years.

If we look at the t2.small:

• A Standard 1 Year Term, no Upfront means I am contracted to pay a minimum of $12.92 per month over 12 months which means I can only use that $12.92 a month on a t2.small but cannot upgrade or downgrade the instance?

• A Standard 1 Year Term, All Upfront means I am contracted to pay $145 up front, which means I can then use a t2.small for 12 months but cannot upgrade or downgrade the instance?

• A Convertible 1 Year Term, no Upfront means I am contracted to pay a minimum of $14.89 per month over 12 months but if I want to upgrade or downgrade the instance I can? What happens if I upgrade? What happens if I downgrade?

• A Convertible 1 Year Term, All Upfront means I am contracted to pay $167 up front... the same above questions I have for this one.

Anyone able to explain it better? Thanks

From what I understand one of the ways to optimize your AWS reserved instance purchases is to buy the smallest unit possible of an instance class and basically use them as building blocks to buy the correct number of instances.

Sites like cloudability have this as part of their pricing matrix... part of the reason is if you need to change sizes of machines downwards for utilization needs that you are not stuck with instances that are uncovered by RI's or if you need to shed workloads you are basically using the smallest unit of an RI rather than having a very expensive instance that goes unused.

From what I understand the math goes something like this:

t2.nano is the smallest t2 instance (this only works in the instance family and does not swap between t/m/c) so buy it right the first time.

To make a t2.small it takes two t2.nano instance purchases.

To make a t2.medium it is two t2.small instance purchases.

So to buy a t2.medium you end up buying four t2.nano instances and this then covers your workload....rather than just buying a single t2.medium.

I have seen a bunch of tools on github that help you plan, check and even purchase instances but I have not seen anything that does the math for you of checking instance types, checking what is not covered by an RI and then suggesting the RI purchases based on the smallest unit.. (Heck if there was even a spreadsheet that one can enter their current inventory and then have it output the units that would be great)

Does anyone here know of a way to do this?