Rightsize Amazon RDS MySQL Read Replica and Primary

SAA-C03 By islever Editorial Last updated

Design cost-optimized storage solutions.

Short answer

B Resize the read replica to a smaller instance size Do not make changes to the primary instance

Question

A company uses an Amazon RDS for MySQL instance. To prepare for end-of-year processing, the company added a read replica to accommodate extra read-only queries from the company's reporting tool. The read replica CPU usage was 60% and the primary instance CPU usage was 60%. After end-of-year activities are complete, the read replica has a constant 25% CPU usage. The primary instance still has a constant 60% CPU usage. The company wants to rightsize the database and still provide enough performance for future growth. Which solution will meet these requirements?
  1. Delete the read replica Do not make changes to the primary instance
  2. Resize the read replica to a smaller instance size Do not make changes to the primary instance Correct Answer
  3. Resize the read replica to a larger instance size Resize the primary instance to a smaller instance size
  4. Delete the read replica Resize the primary instance to a larger instance

Key Takeaways

The question tests cost optimization by identifying which component is over-provisioned (read replica at 25% CPU) versus consistently loaded (primary at 60% CPU). The trap is assuming the primary needs resizing because it is 'busy,' ignoring that 60% is a healthy operating range.

Determine the optimal instance sizing for an Amazon RDS MySQL read replica and primary database to balance cost savings with performance needs. This analysis confirms that resizing the underutilized read replica is the correct approach while leaving the busy primary unchanged.

Common mistake: Many learners choose D, believing the primary instance's constant 60% CPU usage indicates a bottleneck that requires more power, leading them to delete the replica and upgrade the primary instead of right-sizing the idle replica.

Expert Analysis

Why the Answer Is Correct

Option B is the correct solution because the read replica is significantly underutilized at 25% CPU after the peak period, presenting a clear opportunity to reduce costs by downsizing. The primary instance remains at 60% CPU, which is well within acceptable operational limits and does not indicate a need for immediate expansion or right-sizing. Deleting the read replica (Options A and D) would eliminate the ability to offload read queries in the future, contradicting the requirement to support future growth.

Why the Other Options Are Wrong

Option C is incorrect because resizing the read replica to a larger instance increases costs unnecessarily when it is already underutilized, and there is no evidence the primary needs more power. Option A ignores the opportunity to optimize costs on the read replica. Option D removes the scalability feature entirely, forcing the primary to handle all workloads, which risks performance degradation during future spikes and violates the 'future growth' requirement.

Community Comment Notes

The community is split, but the technical consensus favors B. As Oghare noted, the read replica is underutilized and should be resized to save costs while still handling reduced queries. LeonSauveterre emphasized that deleting the replica risks future scalability issues during traffic spikes. Some users like Ghoneam argue for D, focusing on budget reallocation, but this ignores the architectural benefit of maintaining a dedicated read path for future growth.

Exam Strategy

Always check the CPU utilization metrics for both primary and replica instances. If one is low (<30-40%) and the other is moderate/high (40-70%), prioritize right-sizing the low-utilization resource first to save money without impacting performance capacity.

Frequently Asked Questions

Why not resize the primary instance if it's at 60% CPU?

60% CPU is a healthy operating level for a primary database. It indicates the instance is handling its workload efficiently without being overloaded. Right-sizing is typically recommended when usage is consistently above 80-90% or below 20%.

Is it safe to delete the read replica to save costs?

No, because the question explicitly states the company wants to provide enough performance for future growth. Deleting the replica removes the ability to offload read traffic, potentially causing bottlenecks during future peak periods.

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Community Votes

B
60%
D
40%

60% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 7 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 7

  1. Oghare 👍 3
    Answer is B Since the read replica is now underutilized with only 25% CPU usage, it can be resized to a smaller instance to save costs while still handling the reduced read queries. No changes to the primary instance are needed, as it is consistently running at 60% CPU usage, which is manageable, and the read replica will still offload read queries in the future.
  2. Ghoneam 👍 3 Answered D
    D. Delete the read replica Resize the primary instance to a larger instance because no need for read replica after year end still provide enough performance for future growth.
  3. LeonSauveterre 👍 2 Answered B
    Not A or D because they risk future scalability issues during traffic spikes. Moreover, when the end of next year comes, you have to bring back the deleted instance and configure it all over again. As a solutions architect, I wouldn't recommend things like that (personally).
  4. muhammadahmer36 👍 2 Answered B
    Resize the read replica to a smaller instance size Do not make changes to the primary instance
  5. Sarayounisaldossary 👍 1 Answered D
    You should remove an underused resource (read replica). Use that budget to increase primary capacity, solving the CPU load. Even though Option B is “safe” and cost-conscious, it ignores the performance needs of the primary, which is where the real issue lies. Option D is more aligned with AWS best practices and the question's goal of performance + future growth.
Show 2 more comments
  1. Abdullah2004 👍 1
    No clear explanation here and everyone agreed with default answer
  2. aragon_saa 👍 1 Answered B
    Answer is B

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