DBS-C01 Monitoring and Troubleshooting Practice Question
A company uses Amazon RDS for PostgreSQL with Multi-AZ. The primary instance fails and a failover occurs. After failover, the application reports elevated write latency. Which TWO are possible causes?
⚠ Common exam trap
Test-takers frequently confuse Multi-AZ failover with read replica promotion, or assume that automated backups cause performance degradation, when in fact the cold buffer pool is the primary culprit for elevated write latency after failover.
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
✓
The buffer pool is not warm on the new primary
After a Multi-AZ failover, the new primary instance starts with a cold buffer pool (no cached data blocks). PostgreSQL relies on shared buffers to cache frequently accessed data; without a warm cache, every read request must fetch data from disk, which increases I/O and write latency because writes often require reading the affected pages first. This is a known behavior in RDS for PostgreSQL after failover, and it resolves as the buffer pool warms up over time.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A read replica is now promoting to primary
Why it's wrong here
Read replicas are not part of Multi-AZ failover.
- ✓
The buffer pool is not warm on the new primary
Why this is correct
Cold buffer pool increases read I/O.
- ✗
The new primary has a smaller instance size
Why it's wrong here
Multi-AZ replicas must be same size.
- ✗
Automated backups are running on the new primary
Why it's wrong here
Backups are I/O friendly.
- ✓
Application DNS cache still points to the old primary IP
Why this is correct
Connecting to old IP fails or times out.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.