- A
The DMS migration job is not using a source read replica.
Why wrong: Using a read replica can reduce load but not directly cause lag; not having one doesn't increase lag.
- B
The target Cloud SQL instance is not using Premium tier.
Why wrong: Tier affects performance but is not the most likely cause; it can be adjusted.
- C
Large transactions on the source database are causing CDC to fall behind.
Large transactions generate many log records that can overwhelm CDC replication, increasing lag.
- D
The source database has a low transaction log retention period.
Why wrong: Low retention can cause data loss, not increased lag.
Question 160 of 980
PCD Practice Question: Manage a Solution that Can Span Multiple Database Systems
This PCD practice question tests your understanding of manage a solution that can span multiple database systems. 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.
A company is migrating from on-premises SQL Server to Cloud SQL SQL Server using Database Migration Service (DMS). During the continuous migration phase, they notice that the replication lag is increasing. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
✓
Large transactions on the source database are causing CDC to fall behind.
Option C is correct because Database Migration Service (DMS) uses Change Data Capture (CDC) to replicate ongoing changes from the source SQL Server to the target Cloud SQL instance. Large transactions on the source database generate a high volume of log records that the CDC reader must process sequentially. If the transaction volume exceeds the throughput capacity of the DMS replication slot, the lag between the source and target increases, causing the replication lag to grow.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The DMS migration job is not using a source read replica.
Why it's wrong here
Using a read replica can reduce load but not directly cause lag; not having one doesn't increase lag.
- ✗
The target Cloud SQL instance is not using Premium tier.
Why it's wrong here
Tier affects performance but is not the most likely cause; it can be adjusted.
- ✓
Large transactions on the source database are causing CDC to fall behind.
Why this is correct
Large transactions generate many log records that can overwhelm CDC replication, increasing lag.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The source database has a low transaction log retention period.
Why it's wrong here
Low retention can cause data loss, not increased lag.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that replication lag is caused by target-side performance issues (like tier or read replicas), when in fact the bottleneck is almost always the source-side CDC capture rate, especially during large transactions.
Detailed technical explanation
How to think about this question
DMS for SQL Server uses the source database's transaction log as the CDC source, reading log sequence numbers (LSNs) to capture changes. Large transactions (e.g., bulk inserts, index rebuilds) generate many log records that must be read and applied in order, creating a bottleneck if the DMS worker's throughput is exceeded. In real-world scenarios, this often occurs during ETL loads or batch updates, where a single transaction can produce millions of log records, causing the replication lag to spike until the transaction commits and is fully processed.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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Manage a Solution that Can Span Multiple Database Systems — study guide chapter
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FAQ
Questions learners often ask
What does this PCD question test?
Manage a Solution that Can Span Multiple Database Systems — This question tests Manage a Solution that Can Span Multiple Database Systems — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Large transactions on the source database are causing CDC to fall behind. — Option C is correct because Database Migration Service (DMS) uses Change Data Capture (CDC) to replicate ongoing changes from the source SQL Server to the target Cloud SQL instance. Large transactions on the source database generate a high volume of log records that the CDC reader must process sequentially. If the transaction volume exceeds the throughput capacity of the DMS replication slot, the lag between the source and target increases, causing the replication lag to grow.
What should I do if I get this PCD question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jul 4, 2026
This PCD practice question is part of Courseiva's free Google Cloud 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 PCD exam.
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