Documentation and Best Practices

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Reserved Instance benefit: Instance Size Flexibility

EC2 reserved instances as of March 2017 are eligible for an Instance Size Flexibility (ISF) benefit -- allowing the savings of an RI to apply to any size of compute instance inside a family. Reservations you currently own or are planning to buy with attributes of Linux, Regional, and Shared Tenancy will automatically be an ISF reserved instance.

Converting between instance sizes

When our recommendations engine identifies ISF RI’s in your inventory, we assign those ISF reservations a number of units, using a normalization factor, and determine how many RI’s you will need to cover your usage. The normalization factor table below shows how to convert between instances sizes within a family.


For example, if you own an RI that is 2xlarge (16 units), that RI could apply instead to two xlarge (8 units each) or four large (4 units each) instances. You could also use an RI that is 2xlarge (16 units) to apply to one xlarge (8 units) and two large (2 units each) instances.


RI Planner support

Our RI Planner supports recommendations for ISF reservations. We analyze your usage and recommend for ISF-eligible usage that you purchase RI’s for the smallest instance size for that usage’s family. Note that you may need to request an increase to your RI purchase limit from Amazon to accommodate purchasing a high quantity of small instance sizes. You can do so in the AWS Management Console using the following steps:

Select Services.


Select from the Compute options: EC2.


You now will be in the EC2 Dashboard.


From there, select Limits.


Scroll down to Reserved Instances and select Request limit increase.


Billing for ISF RI’s

The billing for ISF RI’s depends on what size instance you run relative to the RI you own. A large instance can now be covered by a smaller size RI, and get a prorated on-demand charge for the remainder of the units for that instance size. For example, if you own a medium RI (2 units) and are running a large instance (4 units), the RI would cover 50% of the cost per hour, and the remaining 50% of the hour would be billed (prorated) at the on-demand rate of the large instance. On the other hand, when you own an RI that is larger than the instance you run, the instance will be covered completely by the RI per hour, but the remaining units unused in that hour (that is, for cases where no other instance in that family is running for that hour) cannot be carried over to apply to instances run at a later time.

Changes to Cloudability with ISF RI’s

The changes you will see in Cloudability Reports are that Usage Quantity now reflects the number of an RI size used to cover the instance you run. The example below shows a single r3.8xlarge instance run over a single day (24 hours), covered by multiple RI’s of differing sizes, with just 1/4 hour of on-demand usage. The Usage Type now contains the instance type of the RI applied to those hours (refer to the Instance Type to see the actual size of the instance).


Here is a more detailed explanation of the numbers in the report above:


Instance Run

RI Used to Cover Instance

Usage Hours

Usage Quantity


Line 1

r3.8xlarge (64 units)

r3.8xlarge (64 units)



The instance was run on-demand for 1/4 of an hour (0.25 hours - the rest was covered by RIs below).

Line 2

r3.8xlarge (64 units)

r3.xlarge (8 units)



5/8 of an hour (0.625 hours) was covered by a smaller RI.

Usage Quantity: 5.00 hours of an r3.xlarge RI (8 units) were used to cover 0.625 hours of a larger r3.8xlarge instance (64 units) -- the math to normalize the different sizes is 5*8=0.625*64.

Line 3

r3.8xlarge (64 units)

r3.2xlarge (16 units)



For 7.000 hours that day, the instance was covered by a smaller RI.

Usage Quantity: 28.00 hours of an r3.2xlarge RI (16 units) were used to cover 7.00 hours of a larger r3.8xlarge instance (64 units) -- the math to normalize the different sizes is 28*16=7*64.

Line 4

r3.8xlarge (64 units)

r3.8xlarge (64 units)



For 16.125 hours that day, the instance was covered by an RI of the same size -- no normalization required.


Note, these changes in Usage Quantity are also reflected in Usage Hours, On Demand Hours, and Reserved Hours -- metrics that now can all be fractional, with RI coverage being applied to a partial hour (some, but not all, of an instance's equivalent hourly units).

Caveats for ISF RI’s are that the ISF benefit 1) applies for the Linux/UNIX option only, 2) does not offer a capacity guarantee because of the Region requirement, and 3) applies within a family only. To move between families, you will need a convertible RI.

Best practices

Our best practice for cases where you do not need capacity guarantee is to modify any AZ-scoped RI’s you own with attribute Linux and Shared Tenancy to have Regional benefit as well. From there, the RI’s will automatically become an ISF RI and benefit from the increased flexibility.

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