Documentation and Best Practices

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Rightsizing FAQ

FAQ

Why is the default recommendation time period 10 days?

Ten days captures the most recent performance trends and is more predictive of future resource use.

How is Idle defined?

Idle for EC2 is the time spent below 2% CPU, on a scale of 1-100. Idle for EBS is percent of hours with zero IOPS.

How do you determine spend?

Spend is instance usage spend for EC2 and RDS, Redshift excludes data transfer, and S3 and EBS are GB Months. 

Why do I see (not set) under State for EBS?

The account may have insufficient permissions (ensure your policy is up to date with the latest permissions), or the resource was active for less than an hour (so may not appear in the Describe Data cache from AWS).

Why do I see "N/A" for lastAttachedTime for unattached EBS volumes?

Volumes will typically show the timestamp that they were last attached to an EC2 instances as well as the volumeID. For volumes that have not been attached during the rightsizing window (the last 10-days by default), however, we will show N/A.

For EC2, do the recommendations take into account burst?

We use baseline performance for burstable resources (e.g. T2 instance types) and do not account for the burst in determining the recommendations. This ensures we make conservative recommendations that won't result in resource ‘clipping.’

For EC2, how do you determine Network and Disk?

AWS does not report EC2 Network and Disk throughput limits. Furthermore, the throughput capacity will vary based on both workload (e.g. sequential versus random read and write) and transfer (e.g. data transfer within a region versus across regions). We use common observed limits across our customer portfolio to approximate capacity.

Why are Spot Instances excluded from rightsizing recommendations?

Our rightsizing engine takes into account your workload (utilization metrics) and the cost to run it (current instance type and on-demand price), and generates a list of recommendations from which you can choose to help save you money. Spot Instances, on the other hand, are already offered at steep discounts; applying rightsizing in these situations results in negligible savings. For this reason, we have chosen to exclude Spot Instances from rightsizing.

Note: When we recommend that you move an EC2 instance type to an instance with no local storage, we incorporate into the recommendation that you add EBS, and account for both in the savings.

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