Maxmemory setting in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of the command to check maxmemory?

You run the Redis command CONFIG GET maxmemory. What is the expected output format?

Redis

CONFIG GET maxmemory

A ["maxmemory"]

B "maxmemory 0"

C ["maxmemory", "0"]

D 0

💡 Hint

Remember Redis returns configuration as an array with key and value.

The CONFIG GET command returns an array with the parameter name and its value as strings.

🧠 Conceptual

intermediate

What happens if maxmemory is set to 0 in Redis?

In Redis, if you set maxmemory to 0, what does it mean for memory usage?

A Redis has no memory limit and can use all available memory.

B Redis will limit memory usage to 0 bytes and refuse writes.

C Redis will use a default memory limit of 100MB.

D Redis will automatically evict keys when memory reaches 0.

💡 Hint

Think about what zero means as a limit.

Setting maxmemory to 0 disables memory limits, so Redis can use all available memory.

📝 Syntax

advanced

Which command correctly sets maxmemory to 100 megabytes?

Choose the correct Redis command to set maxmemory to 100MB.

A CONFIG SET maxmemory 100000000

B CONFIG SET maxmemory 100MB

C CONFIG SET maxmemory 100mb

D CONFIG SET maxmemory 104857600

💡 Hint

Redis expects maxmemory in bytes, not with units.

Redis requires maxmemory value in bytes, so 100MB = 100 * 1024 * 1024 = 104857600 bytes.

❓ optimization

advanced

How to configure Redis to evict least recently used keys when maxmemory is reached?

You want Redis to remove least recently used keys automatically when memory limit is reached. Which command sets this behavior?

A CONFIG SET maxmemory-policy volatile-lru

B CONFIG SET maxmemory-policy allkeys-lru

C CONFIG SET maxmemory-policy noeviction

D CONFIG SET maxmemory-policy allkeys-random

💡 Hint

Look for a policy that evicts any key based on usage.

The allkeys-lru policy evicts the least recently used keys from all keys when memory is full.

🔧 Debug

expert

Why does Redis not evict keys despite maxmemory being set and policy configured?

You set maxmemory to 50MB and maxmemory-policy to allkeys-lru, but Redis never evicts keys and memory usage grows beyond 50MB. What is the most likely cause?

A The maxmemory-policy setting is ignored if maxmemory is zero or not set properly.

B Eviction only happens if keys have expiration times set.

C Redis only evicts keys when memory usage reaches maxmemory plus a safety margin.

D The maxmemory setting is not applied because Redis was not restarted.

💡 Hint

Check if maxmemory is properly set and non-zero.

If maxmemory is zero or not set properly, eviction policies are ignored and Redis won't evict keys.

Practice

1. What does the maxmemory setting in Redis control?

easy

A. The maximum amount of memory Redis can use

B. The maximum number of clients connected

C. The maximum size of a single key

D. The maximum number of commands per second

Solution

1. Step 1: Understand the purpose of maxmemory

   The maxmemory setting limits how much memory Redis can use to avoid overloading the server.

2. Step 2: Compare options with this purpose

   Only the maximum amount of memory Redis can use matches this description, others relate to clients, keys, or commands, which are unrelated.

3. Final Answer:

   The maximum amount of memory Redis can use -> Option A

4. Quick Check:

   maxmemory limits Redis memory use [OK]

Hint: maxmemory always limits Redis memory size [OK]

Common Mistakes:

• Confusing maxmemory with client limits
• Thinking maxmemory limits key size
• Assuming maxmemory limits command rate

2. Which of the following is the correct syntax to set maxmemory to 512 megabytes using CONFIG SET in Redis?

easy

A. CONFIG SET maxmemory 512mb

B. CONFIG SET maxmemory 512MB

C. CONFIG SET maxmemory 512m

D. CONFIG SET maxmemory 512

Solution

1. Step 1: Recall Redis size suffix format

   Redis accepts size suffixes like KB, MB, GB in uppercase for memory settings.

2. Step 2: Check each option's suffix

   CONFIG SET maxmemory 512MB uses 'MB' uppercase which is correct; CONFIG SET maxmemory 512mb uses lowercase 'mb' which is invalid; CONFIG SET maxmemory 512m uses 'm' which is not recognized; CONFIG SET maxmemory 512 has no suffix, so it's bytes, not megabytes.

3. Final Answer:

   CONFIG SET maxmemory 512MB -> Option B

4. Quick Check:

   Use uppercase suffixes like MB for megabytes [OK]

Hint: Use uppercase suffixes like MB, GB for sizes [OK]

Common Mistakes:

• Using lowercase suffixes like mb
• Using incomplete suffix like m
• Omitting suffix and expecting megabytes

3. Given the command CONFIG SET maxmemory 1GB executed on a Redis server, what will be the effect?

medium

A. Redis will limit its memory usage to 1 gigabyte

B. Redis will allow unlimited memory usage

C. Redis will limit memory to 1 byte

D. Redis will throw a syntax error

Solution

1. Step 1: Interpret the command meaning

   The command sets maxmemory to 1GB, meaning Redis should limit memory use to 1 gigabyte.

2. Step 2: Validate the command syntax and effect

   The syntax is correct and Redis supports 'GB' suffix, so it will enforce the memory limit.

3. Final Answer:

   Redis will limit its memory usage to 1 gigabyte -> Option A

4. Quick Check:

   CONFIG SET maxmemory 1GB limits memory to 1GB [OK]

Hint: CONFIG SET maxmemory with GB limits memory to gigabytes [OK]

Common Mistakes:

• Thinking it sets unlimited memory
• Confusing bytes with gigabytes
• Expecting syntax error from correct command

4. You run CONFIG SET maxmemory 100mb on Redis but it returns an error. What is the likely cause?

medium

A. Redis does not support changing maxmemory at runtime

B. maxmemory must be set in bytes only, no suffix allowed

C. The value 100mb is too large for Redis

D. The suffix 'mb' is lowercase and invalid

Solution

1. Step 1: Check Redis suffix case sensitivity

   Redis requires uppercase suffixes like MB, GB for memory sizes.

2. Step 2: Analyze the error cause

   Using lowercase 'mb' causes syntax error; changing to 'MB' fixes it.

3. Final Answer:

   The suffix 'mb' is lowercase and invalid -> Option D

4. Quick Check:

   Use uppercase suffixes for memory sizes [OK]

Hint: Always use uppercase suffixes like MB, GB [OK]

Common Mistakes:

• Assuming maxmemory can't be changed at runtime
• Using lowercase suffix causing errors
• Thinking maxmemory only accepts bytes

5. You want Redis to use at most 2 gigabytes of memory and automatically evict keys when the limit is reached. Which two commands should you run?

hard

A. CONFIG SET maxmemory 2gb and CONFIG SET maxmemory-policy allkeys-random

B. CONFIG SET maxmemory 2GB and CONFIG SET maxmemory-policy noeviction

C. CONFIG SET maxmemory 2GB and CONFIG SET maxmemory-policy allkeys-lru

D. CONFIG SET maxmemory 2048 and CONFIG SET maxmemory-policy volatile-lru

Solution

1. Step 1: Set maxmemory to 2 gigabytes correctly

   Use CONFIG SET maxmemory 2GB with uppercase suffix for 2 gigabytes.

2. Step 2: Choose eviction policy that evicts keys automatically

   allkeys-lru evicts least recently used keys from all keys, enabling automatic eviction.

3. Step 3: Verify other options

   Lowercase '2gb' with allkeys-random uses invalid suffix and random eviction; 2GB with noeviction disables eviction; 2048 (bytes, not GB) with volatile-lru limits to tiny memory and only evicts expiring keys.

4. Final Answer:

   CONFIG SET maxmemory 2GB and CONFIG SET maxmemory-policy allkeys-lru -> Option C

5. Quick Check:

   Set maxmemory + allkeys-lru for eviction [OK]

Hint: Use uppercase GB and allkeys-lru for eviction [OK]

Common Mistakes:

• Using lowercase suffix like '2gb'
• Choosing noeviction policy
• Setting maxmemory without suffix
• Using volatile-lru when all keys need eviction