Memory and storage engine basics (WiredTiger) in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the WiredTiger storage engine in MongoDB?

WiredTiger is the default storage engine in MongoDB that manages how data is stored on disk and cached in memory. It uses compression and supports document-level concurrency control for better performance.

beginner

How does WiredTiger use memory for caching?

WiredTiger uses a cache in RAM to store frequently accessed data and indexes. This cache speeds up read and write operations by reducing disk access.

intermediate

What is document-level locking in WiredTiger?

Document-level locking means WiredTiger locks only the specific document being changed, not the whole collection. This allows many operations to happen at the same time without waiting.

intermediate

Why does WiredTiger use compression for data storage?

WiredTiger compresses data to save disk space and reduce the amount of data read from or written to disk, which improves speed and efficiency.

advanced

What happens if the WiredTiger cache is full?

When the cache is full, WiredTiger writes dirty data (changes) to disk to free up space. This process is called checkpointing and helps keep data safe and memory available.

What is the default storage engine in MongoDB?

A RocksDB

B MMAPv1

C InMemory

D WiredTiger

WiredTiger is the default storage engine in MongoDB since version 3.2.

What type of locking does WiredTiger use to improve concurrency?

A Document-level locking

B Database-level locking

C Collection-level locking

D No locking

WiredTiger uses document-level locking to allow multiple operations on different documents simultaneously.

Why does WiredTiger use compression?

A To reduce disk space and improve speed

B To increase disk space usage

C To slow down data access

D To encrypt data

Compression reduces disk space usage and speeds up data access by reading and writing less data.

What does WiredTiger do when its cache is full?

A Deletes old data

B Writes dirty data to disk (checkpointing)

C Stops accepting writes

D Increases cache size automatically

WiredTiger writes dirty data to disk to free cache space, a process called checkpointing.

What is stored in the WiredTiger cache?

A Only indexes

B Only data

C Frequently accessed data and indexes

D Backup files

The cache holds frequently accessed data and indexes to speed up operations.

Explain how WiredTiger manages memory and storage to improve MongoDB performance.

Think about caching, locking, compression, and writing data safely.

Describe the benefits of document-level locking in WiredTiger.

Compare locking at document level vs collection level.

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