The answer is to use S3DistCp to coalesce files into fewer, larger files. This improves Athena performance because Athena’s engine incurs significant overhead when listing, opening, and reading metadata for thousands of tiny files; by consolidating small files into fewer, larger blocks—ideally 128 MB or more—you reduce the number of S3 GET requests and allow Athena to leverage parallel reads more efficiently. On the AWS Certified Machine Learning Specialty MLS-C01 exam, this concept tests your understanding of how data layout directly impacts query speed, often appearing in scenarios where a partitioned table still runs slowly due to file fragmentation. A common trap is assuming partitioning alone guarantees performance, but without coalescing small files, the overhead of many tiny files can negate the benefits of partitioning. Remember the memory tip: “Partitioning is for pruning, coalescing is for reading”—you need both for optimal Athena performance.
MLS-C01 Data Engineering Practice Question
This MLS-C01 practice question tests your understanding of data engineering. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Network Topology
A data engineer is investigating a slow Athena query on a partitioned table. The table is partitioned by year, month, and day, and the data is stored in S3 with the prefix pattern 'raw/YYYY/MM/DD/'. The engineer runs the above CLI command and sees that there are many small files. Which action would most improve query performance?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use S3DistCp to coalesce files into fewer, larger files.
Athena performs best with larger files. Consolidating small files into fewer, larger files (e.g., using S3DistCp or Glue ETL) reduces the overhead of reading many small files and improves query performance.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Convert the data to columnar format like Parquet or ORC.
Why it's wrong here
Columnar formats help but do not directly address the small files issue.
✓
Use S3DistCp to coalesce files into fewer, larger files.
Why this is correct
Coalescing reduces the number of files, improving query performance.
Related concept
Static NAT maps one inside address to one outside address.
✗
Increase the number of partitions in the Athena DDL.
Why it's wrong here
More partitions without fixing the file size issue may not help.
✗
Add more partitions to reduce the amount of data scanned per query.
Why it's wrong here
More partitions can increase overhead if there are many small files.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related MLS-C01 NAT questions on configuration and troubleshooting.
Data Engineering — This question tests Data Engineering — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Use S3DistCp to coalesce files into fewer, larger files. — Athena performs best with larger files. Consolidating small files into fewer, larger files (e.g., using S3DistCp or Glue ETL) reduces the overhead of reading many small files and improves query performance.
What should I do if I get this MLS-C01 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related MLS-C01 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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This MLS-C01 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 MLS-C01 exam.
Question Discussion
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