Reduce EC2 Data Transfer Costs in Batch Processing

SAA-C03 By islever Editorial Last updated

Design cost-optimized storage solutions. Design cost-optimized compute solutions.

Short answer

C Place all the EC2 instances in the same Availability Zone to eliminate data transfer charges between instances.

Question

A solutions architect is creating an application that will handle batch processing of large amounts of data. The input data will be held in Amazon S3 and the output data will be stored in a different S3 bucket. For processing, the application will transfer the data over the network between multiple Amazon EC2 instances. What should the solutions architect do to reduce the overall data transfer costs?
  1. Place all the EC2 instances in an Auto Scaling group.
  2. Place all the EC2 instances in the same AWS Region.
  3. Place all the EC2 instances in the same Availability Zone. Correct Answer
  4. Place all the EC2 instances in private subnets in multiple Availability Zones.

Key Takeaways

The question tests knowledge of AWS data transfer pricing models, specifically the trap of assuming Region-level placement is sufficient when AZ-level placement is required for free inter-instance traffic.

To minimize data transfer costs for batch processing between EC2 instances and S3, place all instances in the same Availability Zone to eliminate cross-AZ charges.

Common mistake: Placing instances in the same Region (Option B) is the most common mistake; while Region-to-Region transfers are costly, Same-Region but Cross-AZ transfers still incur fees for instance-to-instance communication.

Expert Analysis

Why the Answer Is Correct

Correct answer: C — Placing all EC2 instances in the same Availability Zone ensures that data transfer between the instances is free. While data transfer from EC2 to S3 within the same Region is generally free (if using VPC endpoints or proper routing), the prompt explicitly states the application transfers data "between multiple Amazon EC2 instances." AWS charges for data transfer between instances in different Availability Zones within the same Region. By co-locating them in a single AZ, you avoid these inter-AZ fees entirely.

Why the Other Options Are Wrong

Option A (Auto Scaling) manages capacity but does not inherently optimize data transfer costs unless configured with specific placement constraints. Option B (Same Region) is insufficient because it allows instances to be in different AZs, triggering cross-AZ data transfer charges. Option D (Private subnets in multiple AZs) maximizes high availability but guarantees the highest possible data transfer costs among the options due to inevitable cross-AZ traffic.

Community Comment Notes

Community consensus heavily favors Option C, noting that "data transfer between EC2 instances in the same AZ is free" as per official AWS documentation. Some users questioned if S3 replication affects this, but since S3 is a regional service and the cost driver here is EC2-to-EC2 traffic, AZ placement remains the critical factor for minimizing those specific transfer costs.

Official Reference

https://aws.amazon.com/ec2/pricing/on-demand/

This link opens the official vendor documentation in a new tab.

Exam Strategy

Always distinguish between 'same region' and 'same availability zone' when cost optimization involves data movement between compute resources. If the workload requires low-latency, high-volume internal communication, colocate in a single AZ to save on inter-AZ transfer fees.

Frequently Asked Questions

Is data transfer from EC2 to S3 free?

Data transfer from EC2 to S3 is free within the same Region. However, the question focuses on EC2-to-EC2 transfer, which incurs costs across AZs.

Why not just use the same Region?

Being in the same Region is not enough. If instances are in different AZs, AWS charges for the data transferred between them. Same AZ means zero cost.

More SAA-C03 FAQ →

Community Votes

C
83%
B
17%

83% of anonymous learners picked answer C. 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. ckhemani 👍 3 Answered C
    K. After research it seems C make sense since it is saying data transfer between EC2 and data transfter between two EC2 in AZ is free but not between AZ's. No cost within AZ: Data transfer between EC2 instances in the same AZ is free. Cross-AZ charges: If you transfer data between instances in different Availability Zones, you will be charged for the inter-zone data transfer. Region-based pricing: While data transfer within an AZ is free, data transfer across different regions may also incur charges
  2. ckhemani 👍 2 Answered B
    S3 is repliacted only to 3 AZ's and you can't define AZ while creating the Bucket. If this Question would have said about Directory which is One Zone express where you can define the AZ then it makes sense to select option C. Option B makes sense to configure EC2 in same region since EC2/S3 transfer data between same region is free too.
  3. 78b9037 👍 2 Answered C
    Two reasons: 1. EC2 instances in the same AZ can communicate with each other at no cost. 2. Data transfer between EC2 instances and S3 within the same AZ is free https://aws.amazon.com/blogs/architecture/overview-of-data-transfer-costs-for-common-architectures/
  4. FlyingHawk 👍 1 Answered C
    Data transfer within the zone is free, but cross-zone is not free: https://aws.amazon.com/blogs/architecture/overview-of-data-transfer-costs-for-common-architectures/
  5. viejito 👍 2
    Respuesta correcta C :
Show 2 more comments
  1. aragon_saa 👍 2 Answered C
    Answer is C
  2. sOI852POL 👍 2 Answered C
    C

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