Memory and storage engine basics (WiredTiger) in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

WiredTiger Cache Size Calculation

If a MongoDB instance uses the WiredTiger storage engine on a server with 16 GB of RAM, what is the default WiredTiger cache size allocated by MongoDB?

A 4 GB (25% of RAM)

B 12 GB (75% of RAM)

C 8 GB (50% of RAM minus 1 GB)

D 15 GB (almost all RAM)

💡 Hint

MongoDB reserves some RAM for the OS and other processes, so WiredTiger cache is less than total RAM.

By default, WiredTiger cache size is set to 50% of (RAM - 1 GB) to leave memory for the OS and other MongoDB processes.

❓ query_result

intermediate

WiredTiger Cache Usage Query Result

What is the output of this MongoDB command showing WiredTiger cache statistics? db.serverStatus().wiredTiger.cache

MongoDB

db.serverStatus().wiredTiger.cache

A {"bytes currently in the cache": 104857600, "maximum bytes configured": 8589934592, "tracked dirty bytes in the cache": 5242880}

B {"cache hits": 1000, "cache misses": 500, "cache evictions": 200}

C {"total pages read": 10000, "total pages written": 8000}

D {"connections current": 50, "connections available": 100}

💡 Hint

Look for cache size and bytes info in the WiredTiger cache stats.

The wiredTiger.cache object contains memory usage stats like bytes currently in cache and max bytes configured.

📝 Syntax

advanced

Correct WiredTiger Cache Size Configuration Syntax

Which of the following is the correct way to set WiredTiger cache size to 2 GB in the MongoDB configuration file (mongod.conf)?

A

storage:
  wiredTiger:
    engineConfig:
      cacheSizeGB: 2

B

storage:
  wiredTiger:
    cacheSizeGB: 2

C

wiredTiger:
  engineConfig:
    cacheSizeGB: 2

D

storage:
  engineConfig:
    wiredTiger:
      cacheSizeGB: 2

💡 Hint

Check the correct nesting of keys under storage and wiredTiger.

The cacheSizeGB setting must be under storage.wiredTiger.engineConfig in mongod.conf.

❓ optimization

advanced

Improving WiredTiger Write Performance

Which WiredTiger configuration option can improve write performance by reducing fsync frequency, but increases risk of data loss on crash?

A directoryForIndexes: true

B syncPeriodSecs: 60

C cacheSizeGB: 4

D journalCompressor: snappy

💡 Hint

Look for an option that controls how often data is flushed to disk.

syncPeriodSecs controls how often WiredTiger fsyncs data to disk; increasing it reduces disk writes but risks data loss.

🔧 Debug

expert

Diagnosing WiredTiger Cache Pressure Issue

A MongoDB server using WiredTiger shows frequent cache pressure warnings and slow queries. Which of the following is the most likely cause?

A Journaling is disabled causing data loss

B The database has too many indexes

C MongoDB is running on a single CPU core

D WiredTiger cache size is too small for the working set

💡 Hint

Cache pressure warnings relate to memory usage and cache eviction.

If WiredTiger cache is too small, it cannot hold the working set, causing cache pressure and slow queries.