Caveats on Amazon EC2 Convertible Reserved Instances

Convertible Reserved Instances are a somewhat new type of RIs offered by Amazon. 

At first sight, they offer some really interesting advantages for people with highly varying resources need. You can exchange a Convertible RI (unlike other types of RIs) for any other type of instance (family, OS ...) within the same region. 

For us at Cloudways, is a very good way to get significant discounts for those instances types and sizes of which we have less numbers and are more costly (if at some point, we don't have any, we can just convert the RI to some other instance type and size that we have). 

However during implementation we have found some caveats:

  • First an obvious, is that Convertible RIs are just offered in 3 years format, so you need to be sure that you will keep those resources around for that long.
  • Second and not so obvious is the exchange process in itself. Each reserved instance has a list value (nowhere to be found). As you can exchange Convertible RIs for any other type of instance (including family), this means that you may find situations where it is not possible to exchange a given amount of Convertibles RIs of one type to a whole unit amount of Convertible RIs of another type (i.e. based on list price, your 2 Convertible RIs of instance type X, convert to 3.6 Convertible RIs of type Y) and Amazon will force you to round it up (i.e. 4 Convertible RIs of type Y in the example). This can be a problem if during the three years life span you plan to do plenty of changes, as (due to rounding up) you may end up with a much higher RI count than intended.
  • Additionally, Convertible RIs can't be exchanged for multiple Convertible RIs of different types (i.e. if you have a c4.8xlarge Convertible RI, you can't exchange it for a few c4.large and a few m4.xlarge). You can just exchange all the listed value into another single Convertible instance type.

These points remove part of the charm of Convertible RIs. 

In any case, if you plan using them, I would advice to buy them one by one, as at least you are going to have some extra flexibility in this way.