The answer is a cold buffer pool on the new primary instance. After an RDS PostgreSQL failover, the promoted standby starts with empty shared buffers, meaning no data blocks are cached in memory. PostgreSQL relies on these buffers to serve reads quickly; without a warm cache, every write operation that must first read the affected page from disk incurs additional I/O, directly causing elevated write latency. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of Multi-AZ failover behavior and the performance impact of a cold buffer pool—a common trap is assuming failover is seamless without considering cache state. Remember that writes often require reads first, so a cold cache amplifies latency until the buffer pool warms up naturally over time. Memory tip: “Cold cache, hot writes” — after failover, the cache is cold, making every write feel hot with extra disk I/O.
DBS-C01 Monitoring and Troubleshooting Practice Question
This DBS-C01 practice question tests your understanding of monitoring and troubleshooting. 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 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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
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.
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.
✗
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.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often 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.
Detailed technical explanation
How to think about this question
PostgreSQL uses shared_buffers (typically 75% of instance memory for RDS) to cache data pages; after a failover, the new primary's buffer pool is empty, leading to a 'buffer pool cold start' effect. This causes every write operation to first read the relevant data pages from Amazon EBS storage (which may involve additional I/O if the page is not in the OS page cache), increasing latency until the cache warms up. In practice, this can take minutes to hours depending on workload and instance size, and can be mitigated by pre-warming the buffer pool using extensions like pg_prewarm or by using RDS Proxy to reduce connection churn.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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.
Monitoring and Troubleshooting — This question tests Monitoring and Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this DBS-C01 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: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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