EC2 Savings Plans vs Reserved Instances vs Spot: when to use each

Try 3 questions on this

Pick an answer to see why each option is right or wrong.

1. A batch pipeline runs eight hours a night on EC2 and tolerates interruption well. Finance has a three-year Compute Savings Plan whose commitment is not fully used, and needs to reduce the company's compute spend without wasting that commitment. Which approach will meet these requirements?

   Right. In the game that’s Renown for your Design Cost-Optimized Architectures building.

   Not that one: D is right. In the game your Design Cost-Optimized Architectures building would lose a little Renown, and this question would come back later.

   Your answerA.Keep the pipeline on On-Demand Instances so its usage continues to draw down the Savings Plan commitment nightly.Is the near miss and it is defensible - the commitment is being used - but it keeps an interruptible workload on the most expensive rate it can run at.Your answerB.Move the pipeline to Spot and expect the Savings Plan to discount the Spot price further, compounding both savings.Stacks the two discounts. AWS states that a Savings Plan provides no additional saving on top of Spot pricing.Your answerC.Cancel the Savings Plan before moving the pipeline, because unused commitment is refunded on cancellation.A Savings Plan is a one or three year term commitment. It cannot be cancelled for a refund, so cancelling is not available as a way to avoid waste.The answerD.Move the pipeline to Spot and use the unused commitment elsewhere, because Spot usage draws nothing from a Savings Plan.Both D and A are coherent answers to "do not waste the commitment", and one documented fact decides between them. AWS states the relationship plainly: "Spot Instances are not covered by Savings Plans. If you have a Savings Plans, it does not provide additional savings on top of the savings that you already get from using Spot Instances. Furthermore, your spend on Spot Instances does not apply the commitments in your Compute Savings Plans." So the pipeline on Spot neither gains from the plan nor consumes it - and Spot is cheaper than the On-Demand rate the plan discounts. The commitment is a separate problem, solved by pointing other steady usage at it, which a Compute Savings Plan can do across families, Regions, Fargate and Lambda.

   Written from docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-spot-instances.html

2. A company committed to three-year Reserved Instances for a workload that has since moved from one instance family to another. It wants to keep a commitment discount on the new family for the remainder of the term and to recover value from any reservation it cannot use. Which solution meets these requirements?

   Right. In the game that’s Renown for your Design Cost-Optimized Architectures building.

   Not that one: D is right. In the game your Design Cost-Optimized Architectures building would lose a little Renown, and this question would come back later.

   Your answerA.Let the reservations run to the end of the term unused, since an offering class cannot change after purchase.Offering class is indeed fixed at purchase, which is the reason to use the two exits rather than to accept the loss.Your answerB.Exchange the Standard Reserved Instances for the new family, and sell the Convertible ones in the Reserved Instance Marketplace.Reverses both halves of the table: Standard cannot be exchanged, and Convertible cannot be sold in the Marketplace.Your answerC.Modify every Reserved Instance to the new instance family, because modification covers a family change for both offering classes.Modification is available to both classes for some attributes; an exchange is what changes instance family, and only for Convertibles.The answerD.Exchange the Convertible Reserved Instances for the new family, and sell any Standard Reserved Instances in the Reserved Instance Marketplace.Each offering class has exactly one exit and they are opposites. "A Standard Reserved Instance provides a more significant discount than a Convertible Reserved Instance, but you can't exchange a Standard Reserved Instance. You can exchange Convertible Reserved Instances." On recovery, the table is explicit: a Standard "can be sold in the Reserved Instance Marketplace" and a Convertible "can't be sold in the Reserved Instance Marketplace." So the Convertibles follow the workload to the new family, and the Standards are sold rather than stranded.

   Written from docs.aws.amazon.com/AWSEC2/latest/UserGuide/reserved-instances-types.html

3. A company runs Amazon EKS clusters whose worker nodes are EC2 instances in the m5 and c5 families in two AWS Regions, and some pods run on AWS Fargate. The company will buy one Savings Plan to lower its compute cost and must not commit to more hourly spend than the plan can discount. Which combination of steps should a solutions architect take to meet these requirements? (Select TWO.)

   Right. In the game that’s Renown for your Design Cost-Optimized Architectures building.

   Not that one: B,C is right. In the game your Design Cost-Optimized Architectures building would lose a little Renown, and this question would come back later.

   Your answerA.Buy a smaller commitment now and change its terms later as the Fargate share grows.Relies on a change that is not allowed. The terms of a Savings Plans commitment cannot be changed after purchase.Your answerB.Buy a Compute Savings Plan, because it applies to EC2 instance usage across instance families and Regions and to Fargate usage.Your answerC.Size the hourly commitment from the EC2 worker node and Fargate spend only, leaving out the per-cluster Amazon EKS charge.Your answerD.Size the hourly commitment to include the per-cluster Amazon EKS charge, because a Savings Plan covers every charge of the clusters it runs.Counts a charge the plan never discounts. Amazon EKS charges are not covered by Savings Plans; only the underlying EC2 instances are.Your answerE.Buy an EC2 Instance Savings Plan for the m5 family, because it applies to every instance family that the clusters' nodes use in a Region.Picks the plan tied to one family and Region. An EC2 Instance Savings Plan is a commitment to a specific instance family in a chosen Region, and it does not apply to Fargate.

   Show the full explanation and doc quotes

   Compute Savings Plans "automatically apply to your EC2 instance usage, regardless of instance family (for example, m5, c5, etc.), instance size (for example, c5.large, c5.xlarge, etc.), Region", and "They also apply to your Fargate and Lambda usage." The cluster charge is outside every plan: "Amazon EKS charges will not be covered by Savings Plans, but the underlying EC2 instances will be."

   Written from docs.aws.amazon.com/savingsplans/latest/userguide/plan-types.html