Maxmemory setting in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the maxmemory setting in Redis control?

The maxmemory setting controls the maximum amount of memory Redis can use to store data. When this limit is reached, Redis will start evicting keys based on the eviction policy.

beginner

How can you set the maxmemory limit in Redis?

You can set maxmemory in the Redis configuration file (redis.conf) using the line maxmemory <bytes>, or dynamically at runtime with the command CONFIG SET maxmemory <bytes>.

intermediate

What happens when Redis reaches the maxmemory limit?

When Redis reaches the maxmemory limit, it will evict keys according to the configured eviction policy to free up space for new data. If no eviction policy is set, Redis will return errors on writes.

intermediate

Name two common eviction policies used with Redis maxmemory setting.

Two common eviction policies are allkeys-lru (evicts least recently used keys from all keys) and volatile-lru (evicts least recently used keys only from keys with an expiration).

beginner

Why is setting maxmemory important in Redis?

Setting maxmemory helps prevent Redis from using too much system memory, which could cause the server to slow down or crash. It ensures Redis manages memory efficiently by evicting keys when needed.

What does the Redis maxmemory setting specify?

A Maximum number of keys Redis can store

B Maximum memory Redis can use

C Maximum CPU usage Redis can have

D Maximum network bandwidth Redis can use

The maxmemory setting limits the total memory Redis can use to store data.

Which command sets the maxmemory limit dynamically in Redis?

A CONFIG SET maxmemory <bytes>

B SET maxmemory <bytes>

C CONFIG GET maxmemory

D MEMORY LIMIT <bytes>

The command CONFIG SET maxmemory <bytes> sets the memory limit at runtime.

What happens if Redis reaches the maxmemory limit and no eviction policy is set?

A Redis crashes immediately

B Redis deletes all keys

C Redis automatically increases memory limit

D Redis stops accepting writes and returns errors

Without an eviction policy, Redis returns errors on write commands when memory is full.

Which eviction policy evicts the least recently used keys from all keys in Redis?

A allkeys-lru

B volatile-lru

C noeviction

D random

allkeys-lru evicts least recently used keys from all keys regardless of expiration.

Why should you configure maxmemory in Redis?

A To increase network speed

B To limit CPU usage

C To prevent Redis from using too much memory and crashing the server

D To disable key expiration

Setting maxmemory prevents Redis from using excessive memory that could harm server stability.

Explain what the Redis maxmemory setting does and why it is important.

Think about how Redis manages memory and what happens when it runs out.

Describe how Redis behaves when it reaches the maxmemory limit and how eviction policies affect this behavior.

Consider what Redis does to make space for new data when memory is full.

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