Question 1,021 of 1,663
DBS-C01 Replica Lag Practice Question
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.)
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 Provisioned IOPS on both the primary and standby instances to improve log write throughput
Increasing Provisioned IOPS on both the primary and standby instances improves I/O throughput for writing transaction logs, which is the bottleneck causing replica lag. Higher IOPS reduces the time needed to generate logs on the primary and apply them on the standby. Option A is incorrect because disabling synchronous commit would reduce durability and may still not achieve the required lag reduction. Option B is incorrect since the standby instance size is already equal to the primary; the issue is I/O performance, not compute capacity. Option C is incorrect because reducing batch frequency impacts application responsiveness and does not address the I/O bottleneck.
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.
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Last reviewed: Jun 20, 2026
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