Memory and storage engine basics (WiredTiger) in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check the current storage engine in MongoDB.

MongoDB

db.serverStatus().storageEngine.[1]

Drag options to blanks, or click blank then click option'

Aversion

Bstatus

Cengine

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'version' instead of 'name' returns undefined.

Using 'engine' or 'status' are not valid fields here.

The name field shows the current storage engine used by MongoDB, such as WiredTiger.

2fill in blank

medium

Complete the command to enable the WiredTiger cache size to 2GB in the MongoDB configuration file.

MongoDB

storage:
  wiredTiger:
    engineConfig:
      cacheSizeGB: [1]

Drag options to blanks, or click blank then click option'

A2

B0.5

C4

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Setting cache size too low may reduce performance.

Using strings instead of numbers causes config errors.

Setting cacheSizeGB to 2 allocates 2 gigabytes of RAM for WiredTiger's cache.

3fill in blank

hard

Fix the error in the command to check WiredTiger statistics.

MongoDB

db.serverStatus().wiredTiger.[1]

Drag options to blanks, or click blank then click option'

Acache

Bstats

Cstatus

Dinfo

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'cache' returns only cache info, not full stats.

Using 'status' or 'info' are invalid fields.

The stats field contains detailed WiredTiger statistics like cache usage and page reads.

4fill in blank

hard

Fill both blanks to create a command that lists all collections using WiredTiger storage engine.

MongoDB

db.getCollectionInfos({storageEngine: [1]: [2])

Drag options to blanks, or click blank then click option'

Aname

B"WiredTiger"

Cengine

D"wiredTiger"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'engine' or 'WiredTiger' as keys causes no results.

Using uppercase 'WiredTiger' as value is case sensitive and incorrect.

The name field inside storageEngine is matched against wiredTiger to filter collections using that engine.

5fill in blank

hard

Fill all three blanks to create a command that sets the WiredTiger journal commit interval to 100 milliseconds.

MongoDB

db.adminCommand({setParameter: 1, [1]: [2], [3]: 100})

Drag options to blanks, or click blank then click option'

AwiredTigerEngineRuntimeConfig

B"journalCompressor"

C"commitIntervalMs"

DjournalCommitInterval

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names causes command failure.

Not quoting keys properly causes syntax errors.

The wiredTigerEngineRuntimeConfig parameter accepts a string with runtime settings. Setting "commitIntervalMs" to journalCommitInterval of 100 sets the journal commit interval to 100 milliseconds.

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