Right-Sizing EC2 Instances: The First Step in Cost Optimization
An Auto Scaling group for a background worker runs EC2 instances continuously. Over the last 30 days, CloudWatch shows sustained CPU utilization around 6% with no memory pressure, and queue processing latency meets all SLAs. The team wants to lower monthly cost with minimal risk. What is the best next action?
Quick Answer
The answer is to perform right-sizing by downsizing to a smaller instance family or size and then validating SLAs. This is correct because the workload shows sustained CPU utilization of only 6% with no memory pressure and queue latency already meeting all SLAs, which clearly indicates the current instance is over-provisioned. Right-sizing EC2 instances for cost optimization directly reduces compute spend without introducing risk, as performance requirements are already satisfied. On the SAA-C03 exam, this scenario tests your understanding of the AWS Well-Architected Cost Optimization pillar, specifically the principle of matching capacity to actual demand. A common trap is to suggest purchasing Savings Plans or Reserved Instances first—those lock in waste if the instance is oversized. Instead, always right-size before committing to discounts. Memory tip: “6% CPU? Downsize first, commit later.”
⚠ Common exam trap
Candidates often assume Spot Instances are always the cheapest option, but they ignore the risk of interruption for a continuously running workload where SLAs must be met, making right-sizing the safer and more appropriate first step.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Perform right sizing by downsizing to a smaller instance family/size and validate SLAs
The current instance type is over-provisioned, as sustained CPU utilization is only 6% with no memory pressure and all SLAs are met. Right-sizing to a smaller instance family or size directly reduces compute cost while maintaining performance, making it the lowest-risk, cost-optimization action. This aligns with the AWS Well-Architected Framework's cost optimization pillar, which recommends matching instance capacity to actual workload requirements.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Increase the instance size to reduce CPU throttling risk
Why it's wrong here
The metrics show sustained low CPU with no reported throttling risk. Increasing size would likely increase cost immediately without addressing an existing performance problem.
- ✓
Perform right sizing by downsizing to a smaller instance family/size and validate SLAs
Why this is correct
Right sizing uses actual utilization to remove overprovisioning. With low CPU and no memory pressure and SLAs already met, downsizing (while validating under load and during a controlled rollout) is the safest way to reduce waste.
- ✗
Switch the group to Spot Instances to reduce cost without changing instance sizing
Why it's wrong here
Spot could reduce compute price, but it introduces interruption risk and additional operational complexity. When the primary waste is clearly overprovisioned capacity, downsizing is the more direct and lower-risk cost optimization.
- ✗
Buy Reserved Instances with a long term commitment before making any sizing changes
Why it's wrong here
Purchasing capacity commitments can reduce unit price, but it does not fix the cost waste from running much larger-than-needed instances. Since the instance utilization indicates overprovisioning, right sizing should come first to maximize savings with minimal risk.
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Same concept, more angles
2 more ways this is tested on SAA-C03
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A team runs an EC2-based API on a single Auto Scaling group (ASG). Over the last month, they observed: - Average CPU utilization is ~15%. - p95 latency is stable and within the performance target. - The attached EBS volumes are gp3, provisioned with high baseline IOPS/throughput “just to be safe,” but CloudWatch shows consistently low utilization of those provisioned IOPS/throughput limits. They want to reduce monthly cost while maintaining current performance. Which action is the best cost-optimized choice?
medium- A.Stop resizing EBS and only scale out the ASG during peak traffic, because changing EBS performance settings risks latency spikes.
- ✓ B.Right-size both the compute and the gp3 volumes: reduce the EC2 instance size (via the ASG launch template/desired capacity configuration) and update gp3 IOPS/throughput settings to match observed utilization while keeping p95 latency targets.
- C.Switch the instances to EC2 Spot immediately, because Spot always lowers costs without adding operational risk or affecting performance.
- D.Move the workload to a larger instance class and keep the gp3 settings unchanged to avoid operational tuning work.
Why B: The workload is over-provisioned in both compute and storage. Average CPU is only 15%, so a smaller instance size can handle the load without affecting p95 latency. The gp3 volumes have high baseline IOPS/throughput that are never used, so reducing them to match actual utilization directly lowers costs without performance risk.
Variation 2. CloudWatch metrics show your EC2 instances have average CPU utilization around 10% with stable performance over several weeks. The application does not require additional headroom right now. What is the most effective cost-optimization action?
easy- ✓ A.Right-size the instances to a smaller size that matches the observed utilization
- B.Increase the Auto Scaling desired capacity to add more instances
- C.Switch to Spot Instances immediately even though interruptions would impact users
- D.Disable detailed monitoring to reduce CPU usage from the monitoring agent
Why A: Right-sizing EC2 instances to match observed utilization is the most effective cost-optimization action because the current instances are over-provisioned (average CPU at 10%). By selecting a smaller instance type that aligns with the actual workload, you reduce hourly costs without impacting performance, as the application has stable behavior and no need for headroom.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SAA-C03 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SAA-C03 exam.