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An example: an AWS recommendation features an over-provisioned CPU on a large and expensive instance, and our investigation has shown us that the key current performance indicatoris Disk Throughput.

We would start in this case by checking what the Disk Throughput requirement is in the recommendation (based on its current/observed performance) - both throughput rate and IOPS count should be considered.

This would likely involve one disk being set up as a RAID0 array with multiple volumes to meet requirements, due to the way AWS storage is set up (e.g. GP3 volumes have fixed performance values regarding IOPS and throughput). It is important that we also check the Design restrictions and the recommendation here to see which types of storage can be recommended, and which type is being recommended.

The throughput requirement will likely drive a particular type of disk, or a particular number of volumes in one or more RAID0 arrays. Additionally, all drives for a particular node should be accounted for, so nodes with multiple drives need to take all their recommended drives into account when fitting these on a particular instance.

With this information in mind, we can check the recommended instance family’s performance metrics for some key details: does a smaller version of the same instance not support the required number of drives, e.g.?

We can also check similar, lower performance instance ranges: does a cheaper instance family not support the required storage type, for example? Maybe GP3 drives are required, but the cheaper instance family only supports GP2.

Another thing to consider is Regional Availability - perhaps there is a suitable instance available on the platform that is cheaper and smaller but is unavailable in the region the Design is set to.

In this example, we might discover that the recommendation requires a total of 12 GP3 volumes to cover all its drives and their throughput needs. When we examine the AWS offerings, we see that the instance recommended supports up to 16 GP3 volumes, but the instance immediately below it in the same family only supports up to 8 GP3 volumes. Thus, the lower performance instance is not fit for purpose, while the higher performance instance is.