C100DBA Replication Practice Question
A DBA is adding a new secondary to an existing replica set. The data set is 2 TB, and the network bandwidth between the new member and the primary is 100 Mbps. The DBA wants to minimize the impact on the primary during the initial sync. Which approach should the DBA take?
⚠ Common exam trap
The trap here is assuming that changing replica set settings like priority or hidden status will reduce the primary's load during initial sync, but those settings do not affect the data transfer itself.
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
✓
Seed the new member by copying the data files from an existing secondary while it is offline, then start it with the same replica set configuration.
Seeding a new member by copying data files from an existing secondary (taken offline) is an efficient way to add a member without burdening the primary with a full initial sync over a slow network. This method leverages local copy speed and avoids consuming primary resources, which is critical when network bandwidth is limited.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure the new member as a hidden member to reduce its replication load.
Why it's wrong here
Hidden members still perform initial sync and replicate data; they are just invisible to clients. Configuring the new member as hidden does not reduce the load on the primary or speed up the sync. It only affects client read routing and election eligibility. The primary still serves the initial sync data.
- ✗
Increase the oplog size on the primary to accommodate the initial sync.
Why it's wrong here
Increasing the oplog size on the primary does not speed up the initial sync or reduce its impact. The initial sync copies all data and then applies oplog entries. A larger oplog may help if the sync takes long and the oplog wraps, but it does not address the bandwidth constraint or the primary's load. It is not a targeted solution.
- ✗
Set the new member's priority to 0 during the initial sync to prevent it from becoming primary.
Why it's wrong here
Setting priority to 0 prevents the new member from being elected primary, which is good practice during initial sync, but it does not minimize the impact on the primary. The initial sync still pulls data from the primary, consuming network and I/O. This setting addresses election eligibility, not sync performance.
- ✓
Seed the new member by copying the data files from an existing secondary while it is offline, then start it with the same replica set configuration.
Why this is correct
Copying data files from an existing secondary (while it is offline) is a valid method to seed a new member. This avoids the primary having to serve the entire initial sync, distributing the load. It is faster than syncing over a slow network and reduces impact on the primary. The new member must be started with the same replica set name and oplog sufficient to catch up.
About these practice questions
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official MongoDB exam blueprint
This C100DBA practice question is part of Courseiva's free MongoDB 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 C100DBA exam.