The answer is to increase the transaction log retention period on the source database and use a larger DMS replication instance. This directly resolves DMS CDC failures during migration due to transaction log cleanup, because the error occurs when DMS falls behind during peak hours and the source cleans its logs before DMS can capture the changes; extending retention buys DMS more time to process the backlog, while a larger instance boosts its processing capacity to keep pace with high-volume writes. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of DMS ongoing replication bottlenecks and the interplay between source database configuration and instance sizing—a common trap is to suggest network upgrades like Direct Connect, which don’t fix the log retention issue, or to decrease retention, which worsens the failure. Remember the mnemonic “Logs Lag, Lift Load”: when logs lag behind, lift the retention window and load up on a bigger instance.
DBS-C01 Deployment and Migration Practice Question
This DBS-C01 practice question tests your understanding of deployment and migration. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A company is migrating a 3 TB PostgreSQL database from on-premises to Amazon RDS for PostgreSQL. The migration must have minimal downtime. The on-premises network bandwidth to AWS is 1 Gbps. The database supports a critical application that cannot be stopped for more than 30 minutes. The migration team has set up AWS DMS with ongoing replication using CDC. The initial full load is expected to take 8 hours. During the migration, the DMS task fails with an error indicating that the source database's transaction log is being cleaned before DMS can capture changes. The source database has a transaction log retention period of 24 hours. The migration team notices that the CDC phase falls behind during peak hours. What should the team do to complete the migration successfully within the downtime window?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Increase the transaction log retention period on the source database and use a larger DMS replication instance
The issue is that DMS cannot keep up with changes during peak hours, causing the transaction logs to be cleaned before DMS reads them. Increasing the source database's transaction log retention period gives DMS more time to catch up. Using a larger DMS replication instance will increase processing capacity. Option C is the best combination. Option A (Snowball) would take too long. Option B (Direct Connect) doesn't solve the log retention issue. Option D (decrease retention) makes the problem worse.
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.
✗
Use AWS Snowball to transfer the full database and then use DMS for CDC
Why it's wrong here
Snowball transfer would take days, exceeding the downtime window.
✗
Increase the network bandwidth using AWS Direct Connect
Why it's wrong here
Network bandwidth is not the bottleneck; DMS processing capacity is.
✓
Increase the transaction log retention period on the source database and use a larger DMS replication instance
Static NAT maps one inside address to one outside address.
✗
Decrease the transaction log retention period to free up space
Why it's wrong here
Decreasing retention would exacerbate the problem.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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 DBS-C01 NAT questions on configuration and troubleshooting.
Deployment and Migration — This question tests Deployment and Migration — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Increase the transaction log retention period on the source database and use a larger DMS replication instance — The issue is that DMS cannot keep up with changes during peak hours, causing the transaction logs to be cleaned before DMS reads them. Increasing the source database's transaction log retention period gives DMS more time to catch up. Using a larger DMS replication instance will increase processing capacity. Option C is the best combination. Option A (Snowball) would take too long. Option B (Direct Connect) doesn't solve the log retention issue. Option D (decrease retention) makes the problem worse.
What should I do if I get this DBS-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 DBS-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.
About these practice questions
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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 company is migrating a 5 TB PostgreSQL database from on-premises to Amazon RDS for PostgreSQL. The database has a 24/7 uptime requirement and must be migrated with minimal downtime. The on-premises network bandwidth is 100 Mbps. The migration must be completed within 48 hours. The team has chosen to use AWS Database Migration Service (DMS) with ongoing replication from a change data capture (CDC) source. After setting up the source endpoint, target endpoint, and replication instance, the initial full load takes 30 hours. However, during the CDC phase, the target falls behind by over 2 hours and continues to lag. The replication instance is a dms.c5.large (2 vCPUs, 4 GB memory). The source database is heavily utilized with frequent updates. What should the team do to reduce the CDC lag and meet the migration deadline?
hard
✓ A.Increase the replication instance size to dms.c5.2xlarge.
B.Enable Multi-AZ on the replication instance for better performance.
C.Split the migration into multiple DMS tasks for parallel processing.
D.Disable ongoing replication and use a one-time full load migration.
Why A: Option A is correct: increasing the replication instance size to dms.c5.2xlarge provides more CPU and memory, which can handle higher transaction throughput. Option B is wrong because splitting into multiple tasks adds overhead and may not help with a single source. Option C is wrong because DMS does not use Multi-AZ. Option D is wrong because disabling ongoing replication would cause data loss.
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This DBS-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 DBS-C01 exam.
Question Discussion
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