Memory and storage engine basics (WiredTiger) in MongoDB

Introduction

Memory and storage engines help MongoDB save and manage your data efficiently. WiredTiger is the default engine that balances speed and storage.

When you want fast access to your data with good compression to save disk space.

When running MongoDB on a server with limited memory but need good performance.

When you want to handle many users reading and writing data at the same time.

When you want to use features like document-level locking for better concurrency.

Syntax

MongoDB

storage:
  engine: wiredTiger
  wiredTiger:
    engineConfig:
      cacheSizeGB: <number>
    collectionConfig:
      blockCompressor: <compressor_type>

This is part of the MongoDB configuration file (mongod.conf).

You can set cache size and compression type to control memory and storage behavior.

Examples

Basic setup to use WiredTiger as the storage engine.

MongoDB

storage:
  engine: wiredTiger

Set WiredTiger cache size to 2 GB to control memory usage.

MongoDB

storage:
  engine: wiredTiger
  wiredTiger:
    engineConfig:
      cacheSizeGB: 2

Use Snappy compression to reduce disk space used by collections.

MongoDB

storage:
  engine: wiredTiger
  wiredTiger:
    collectionConfig:
      blockCompressor: snappy

Sample Program

This configuration tells MongoDB to use WiredTiger with 1 GB cache and zlib compression for collections.

MongoDB

# This is a mongod.conf snippet to configure WiredTiger
storage:
  engine: wiredTiger
  wiredTiger:
    engineConfig:
      cacheSizeGB: 1
    collectionConfig:
      blockCompressor: zlib

Important Notes

WiredTiger uses RAM as a cache to speed up data access.

Compression helps save disk space but may use more CPU.

You can tune cache size based on your server's memory to balance speed and resource use.

Summary

WiredTiger is MongoDB's default storage engine that balances speed and storage.

You configure it in mongod.conf by setting engine, cache size, and compression.

Proper tuning helps MongoDB run efficiently on your hardware.

Practice

1. What is the default storage engine used by MongoDB that balances speed and storage?

easy

A. WiredTiger

B. MMAPv1

C. InMemory

D. RocksDB

Solution

1. Step 1: Recall MongoDB default engine

   MongoDB uses WiredTiger as its default storage engine since version 3.2 for better performance and compression.

2. Step 2: Compare options

   MMAPv1 is older, InMemory is for volatile data, RocksDB is not default in MongoDB.

3. Final Answer:

   WiredTiger -> Option A

4. Quick Check:

   Default engine = WiredTiger [OK]

Hint: Default MongoDB engine is WiredTiger [OK]

Common Mistakes:

• Confusing MMAPv1 as default
• Thinking InMemory is default
• Assuming RocksDB is default

2. Which configuration file setting correctly specifies WiredTiger as the storage engine in mongod.conf?

easy

A. storage:
engine: wiredtiger

B. storage:
engine: WT

C. storage:
engine: wiredTiger

D. storage:
engine: WiredTiger

Solution

1. Step 1: Check correct case for engine name

   The engine name is case-sensitive and must be 'wiredTiger' with lowercase 'w' and capital 'T'.

2. Step 2: Validate YAML syntax

   storage: engine: WT uses unrecognized abbreviation; C uses all lowercase; D uses incorrect capitalization.

3. Final Answer:

   storage: engine: wiredTiger -> Option C

4. Quick Check:

   Correct engine name is wiredTiger [OK]

Hint: Use exact 'wiredTiger' (lowercase w, capital T) in config [OK]

Common Mistakes:

• Using all lowercase 'wiredtiger'
• Using 'WiredTiger' with capital W
• Using abbreviations like 'WT'

3. Given this mongod.conf snippet:

storage:
  engine: WiredTiger
  wiredTiger:
    engineConfig:
      cacheSizeGB: 2

What does the cacheSizeGB setting control?

medium

A. The disk space allocated for WiredTiger files

B. The amount of RAM WiredTiger uses for caching data

C. The number of CPU cores WiredTiger uses

D. The network bandwidth limit for WiredTiger

Solution

1. Step 1: Understand cacheSizeGB meaning

   cacheSizeGB sets how much RAM WiredTiger uses to cache data in memory for faster access.

2. Step 2: Eliminate other options

   It does not control disk space, CPU cores, or network bandwidth.

3. Final Answer:

   The amount of RAM WiredTiger uses for caching data -> Option B

4. Quick Check:

   cacheSizeGB = RAM cache size [OK]

Hint: cacheSizeGB sets RAM cache size for WiredTiger [OK]

Common Mistakes:

• Thinking it controls disk space
• Confusing with CPU or network settings
• Ignoring memory tuning importance

4. You set cacheSizeGB: 0.5 in mongod.conf but MongoDB starts with poor performance. What is the likely cause?

medium

A. cacheSizeGB must be at least 1 GB

B. cacheSizeGB cannot be a decimal value

C. cacheSizeGB must be set under storage.engine directly

D. cacheSizeGB is ignored and does not affect startup

Solution

1. Step 1: Check WiredTiger cache size limits

   WiredTiger performs poorly if cacheSizeGB is less than 1 GB.

2. Step 2: Validate config placement and value type

   Decimal values are allowed; placement under wiredTiger.engineConfig is correct; it is not ignored.

3. Final Answer:

   cacheSizeGB must be at least 1 GB -> Option A

4. Quick Check:

   Minimum cacheSizeGB = 1 GB [OK]

Hint: cacheSizeGB minimum is 1 GB for good performance [OK]

Common Mistakes:

• Using decimal values thinking they are invalid
• Placing cacheSizeGB under wrong config section
• Assuming cacheSizeGB is optional for startup

5. You want to optimize MongoDB performance on a server with 16 GB RAM. Which cacheSizeGB setting is best for WiredTiger, considering MongoDB and OS need memory?

hard

A. Set cacheSizeGB to 14 to maximize cache

B. Set cacheSizeGB to 16 to use all RAM for cache

C. Set cacheSizeGB to 2 because more cache slows down MongoDB

D. Set cacheSizeGB to 8 to leave memory for OS and other processes

Solution

1. Step 1: Understand memory needs

   MongoDB needs memory for cache and other operations; OS and other apps also need RAM.

2. Step 2: Choose balanced cache size

   Setting cacheSizeGB to about half or slightly more (8 GB) leaves enough RAM for OS and avoids swapping.

3. Step 3: Evaluate other options

   14 or 16 GB risks OS starvation; 2 GB is too low for good performance.

4. Final Answer:

   Set cacheSizeGB to 8 to leave memory for OS and other processes -> Option D

5. Quick Check:

   Balance cache and OS memory = 8 GB [OK]

Hint: Use about half RAM for cache to keep system stable [OK]

Common Mistakes:

• Assigning all RAM to cache causing OS issues
• Setting too low cacheSizeGB hurting performance
• Ignoring OS memory needs