- A
Restart the migration with a new full load and use a smaller task.
Why wrong: Does not address the root cause.
- B
Increase the undo retention period and undo tablespace size on the source Oracle database.
Larger undo retention prevents snapshot too old errors.
- C
Enable supplemental logging on the source and use a larger DMS instance.
Why wrong: Supplemental logging does not affect undo.
- D
Reduce the transaction volume by pausing non-essential transactions during CDC.
Why wrong: Not feasible in production.
Question 272 of 1,730
Fix Oracle Snapshot Too Old Error in AWS DMS CDC
This DBS-C01 practice question tests your understanding of management and operations. 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 an on-premises Oracle database to Amazon RDS for Oracle using AWS DMS. The source database is 2 TB and has high transaction volume. The migration needs minimal downtime. Currently, full load completes but CDC task fails with 'ORA-1555: snapshot too old' errors. What should the DBA do to resolve this?
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
✓
Increase the undo retention period and undo tablespace size on the source Oracle database.
The 'ORA-1555: snapshot too old' error occurs when Oracle's undo data for a read-consistent view is overwritten before DMS can read it. Increasing the undo retention period and undo tablespace size on the source database ensures that undo data is preserved longer, allowing DMS's CDC task to read consistent data without errors. Option B is correct because it directly addresses the root cause. Option A would restart the full load but not fix the CDC failure. Option C (supplemental logging and larger DMS instance) does not resolve undo retention issues. Option D (reducing transactions) is not practical and may cause data loss. Therefore, the correct solution is to increase undo retention and tablespace size.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Restart the migration with a new full load and use a smaller task.
Why it's wrong here
Does not address the root cause.
- ✓
Increase the undo retention period and undo tablespace size on the source Oracle database.
Why this is correct
Larger undo retention prevents snapshot too old errors.
Related concept
Standard ACLs match source addresses.
- ✗
Enable supplemental logging on the source and use a larger DMS instance.
Why it's wrong here
Supplemental logging does not affect undo.
- ✗
Reduce the transaction volume by pausing non-essential transactions during CDC.
Why it's wrong here
Not feasible in production.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
- Standard ACLs match source addresses.
- Extended ACLs can match source, destination, protocol and ports.
- The first matching ACL entry is used.
- There is usually an implicit deny at the end.
TExam Day Tips
- Check inbound versus outbound direction.
- Read the ACL from top to bottom.
- Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related DBS-C01 ACL questions on filtering logic and placement.
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FAQ
Questions learners often ask
What does this DBS-C01 question test?
Management and Operations — This question tests Management and Operations — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Increase the undo retention period and undo tablespace size on the source Oracle database. — The 'ORA-1555: snapshot too old' error occurs when Oracle's undo data for a read-consistent view is overwritten before DMS can read it. Increasing the undo retention period and undo tablespace size on the source database ensures that undo data is preserved longer, allowing DMS's CDC task to read consistent data without errors. Option B is correct because it directly addresses the root cause. Option A would restart the full load but not fix the CDC failure. Option C (supplemental logging and larger DMS instance) does not resolve undo retention issues. Option D (reducing transactions) is not practical and may cause data loss. Therefore, the correct solution is to increase undo retention and tablespace size.
What should I do if I get this DBS-C01 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related DBS-C01 ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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Last reviewed: Jun 20, 2026
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