C100DBA Server Administration Practice Question
In a dedicated server environment, what is the primary reason an administrator would choose to manually decrease the 'storage.wiredTiger.engineConfig.cacheSizeGB' below the default value?
⚠ Common exam trap
Candidates assume the default cache size is always best, ignoring that on shared hosts, MongoDB's aggressive memory usage can trigger OS-level swapping, severely degrading overall system performance.
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
✓
To accommodate other processes or containers running on the same host.
Effective memory management is the core of MongoDB server administration. While the default cache size is usually optimal, administrators must adjust it when the server hosts other memory-intensive processes. If the operating system or other tools are starved for RAM, it can lead to excessive swapping and system instability, which harms database performance.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
To increase the speed of BSON document compression.
Why it's wrong here
WiredTiger cache size does not directly correlate with compression speed. Compression is a CPU-intensive task performed when data is written to or read from the disk. Reducing the cache size actually increases disk I/O, which could potentially slow down the overall throughput of compressed data operations.
- ✗
To allow more memory for the filesystem cache to store indexes.
Why it's wrong here
WiredTiger stores both data and indexes in its internal cache. While the filesystem cache is also used, reducing the internal cache size forces more data to be evicted to disk, which usually decreases performance. This is not a standard reason to decrease the cache unless the internal cache is misconfigured.
- ✗
To reduce the size of the oplog on the primary node.
Why it's wrong here
The oplog size is a separate configuration parameter ('replication.oplogSizeMB') and is not determined by the WiredTiger cache size. Reducing the cache will not affect how much historical operation data is stored in the oplog, although it might impact the performance of tailing the oplog by secondaries.
- ✓
To accommodate other processes or containers running on the same host.
Why this is correct
The most common reason to decrease the cache size is to ensure that other applications, such as monitoring agents, backup tools, or other database instances, have sufficient RAM to function. Without this adjustment, the mongod process might consume too much memory, leading to OOM (Out of Memory) kills.
About these practice questions
This C100DBA question is part of Courseiva's 222-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
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.