The answer is to increase the Provisioned IOPS on both the primary and standby instances. This is correct because replica lag in RDS PostgreSQL Multi-AZ is often a storage I/O bottleneck, not a compute or network issue; the primary must write transaction logs to disk quickly, and the standby must replay them with equal speed, so boosting IOPS on both sides directly reduces the time spent on log writes and application. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding that high write throughput can saturate the io1 volume’s IOPS limit, causing the standby to fall behind even when instances are identical—a common trap is to assume the standby needs more compute or to disable durability features like synchronous commit, which risks data loss. Remember the memory tip: “Logs lag on low IOPS—boost both boxes to beat the bottleneck.”
DBS-C01 Monitoring and Troubleshooting Practice Question
This DBS-C01 practice question tests your understanding of monitoring and troubleshooting. 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 runs a critical application on Amazon RDS for PostgreSQL with Multi-AZ deployment. The database is a db.r5.xlarge instance with 500 GB of Provisioned IOPS (io1) storage. The application writes a large volume of time-series data every 5 minutes. Recently, the team noticed that the replica lag between the primary and standby instance has increased to over 10 seconds during peak write periods. The application requires that the standby be within 5 seconds of the primary for disaster recovery purposes. The team has already verified that network latency between AZs is minimal, and the standby instance is the same size as the primary. The CloudWatch metric 'TransactionLogsDiskUsage' shows that the primary is generating logs at a high rate. Which combination of actions should the database administrator take to reduce the replica lag? (Choose the best option.)
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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
✓
Increase the Provisioned IOPS on both the primary and standby instances to improve log write throughput
Option B is correct because increasing the storage IOPS on both instances reduces the time to write transaction logs to disk, which is the bottleneck. The primary generates logs, and the standby applies them; both need fast I/O. Option A is insufficient because disabling synchronous commit might cause data loss. Option C is incorrect because the standby is already the same size; increasing its size alone may not help if the bottleneck is I/O. Option D is incorrect because reducing the batch size reduces write volume, affecting application performance.
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.
✗
Disable synchronous commit on the primary to reduce log generation
Why it's wrong here
Disabling synchronous commit can lead to data loss in case of a crash.
✗
Change the standby instance to a larger instance type (e.g., db.r5.2xlarge) to improve apply throughput
Why it's wrong here
The bottleneck is I/O, not CPU/memory; increasing instance size may not help.
✗
Reduce the batch write frequency to every 10 minutes to lower the log generation rate
Why it's wrong here
This would affect the application's data freshness requirements.
✓
Increase the Provisioned IOPS on both the primary and standby instances to improve log write throughput
Why this is correct
Higher IOPS reduces the time to flush transaction logs, reducing replica lag.
Clue confirmation
The clue words "best", "primary" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
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 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.
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.
Monitoring and Troubleshooting — This question tests Monitoring and Troubleshooting — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Increase the Provisioned IOPS on both the primary and standby instances to improve log write throughput — Option B is correct because increasing the storage IOPS on both instances reduces the time to write transaction logs to disk, which is the bottleneck. The primary generates logs, and the standby applies them; both need fast I/O. Option A is insufficient because disabling synchronous commit might cause data loss. Option C is incorrect because the standby is already the same size; increasing its size alone may not help if the bottleneck is I/O. Option D is incorrect because reducing the batch size reduces write volume, affecting application performance.
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.
Are there clue words in this question I should notice?
Yes — watch for: "best", "primary". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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