Memory usage analysis (INFO memory) in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What command in Redis shows detailed memory usage information?

The INFO memory command displays detailed memory usage statistics in Redis.

beginner

What does the used_memory field represent in INFO memory output?

It shows the total number of bytes allocated by Redis for its own data structures and buffers.

intermediate

Explain the difference between used_memory and used_memory_rss in Redis memory info.

used_memory is the memory Redis uses internally, while used_memory_rss is the actual memory allocated by the operating system (Resident Set Size).

intermediate

What does mem_fragmentation_ratio indicate in Redis memory info?

It shows how much memory fragmentation exists. A value close to 1 means low fragmentation, higher values mean more wasted memory.

beginner

Why is monitoring total_system_memory important when analyzing Redis memory usage?

Because it helps understand Redis memory usage relative to the total system memory, which is important for avoiding out-of-memory issues.

Which Redis command provides memory usage details?

A INFO memory

B MEMORY USAGE

C GET memory

D SHOW MEMORY

The INFO memory command shows detailed memory statistics in Redis.

What does a mem_fragmentation_ratio greater than 1 indicate?

A Low memory fragmentation

B Redis is using less memory than allocated

C High memory fragmentation

D Redis is out of memory

A ratio above 1 means memory fragmentation is present, causing wasted memory.

What is the difference between used_memory and used_memory_rss?

A <code>used_memory</code> is OS memory, <code>used_memory_rss</code> is Redis internal memory

B <code>used_memory</code> is Redis internal memory, <code>used_memory_rss</code> is OS memory

C Both are the same

D They represent CPU usage

used_memory is Redis internal allocation; used_memory_rss is actual OS memory used.

Why should you monitor total_system_memory in Redis?

A To know Redis version

B To monitor CPU load

C To check network usage

D To avoid exceeding system memory limits

Knowing total system memory helps prevent Redis from causing out-of-memory errors.

Which field shows the peak memory Redis has used since start?

A used_memory_peak

B used_memory

C mem_fragmentation_ratio

D total_system_memory

used_memory_peak shows the highest memory usage Redis had since it started.

Describe the key memory metrics provided by the Redis INFO memory command and what they mean.

Think about internal memory, OS memory, peak usage, fragmentation, and system limits.

Explain why memory fragmentation matters in Redis and how you can identify it using INFO memory.

Focus on the fragmentation ratio and its effect.

Practice

1. What does the used_memory field in the INFO memory command represent in Redis?

easy

A. The total number of keys stored in Redis

B. The amount of memory Redis is currently using in bytes

C. The peak memory usage recorded since Redis started

D. The fragmentation ratio of Redis memory

Solution

1. Step 1: Understand the INFO memory output

   The INFO memory command shows various memory stats about Redis.

2. Step 2: Identify used_memory meaning

   used_memory specifically shows how many bytes Redis is currently using in RAM.

3. Final Answer:

   The amount of memory Redis is currently using in bytes -> Option B

4. Quick Check:

   used_memory = current memory used [OK]

Hint: Remember: 'used_memory' means current bytes used [OK]

Common Mistakes:

• Confusing 'used_memory' with total keys count
• Mixing 'used_memory' with peak memory
• Thinking it shows fragmentation ratio

2. Which of the following is the correct Redis CLI command to check memory usage details?

easy

A. GET MEMORY INFO

B. MEMORY USAGE

C. SHOW MEMORY

D. INFO memory

Solution

1. Step 1: Recall Redis commands for memory info

   The Redis command to get detailed memory info is INFO memory.

2. Step 2: Check other options

   MEMORY USAGE exists but returns usage for a single key, not overall memory info. The others are invalid commands.

3. Final Answer:

   INFO memory -> Option D

4. Quick Check:

   Use INFO memory to get memory stats [OK]

Hint: Use 'INFO memory' to get full memory stats [OK]

Common Mistakes:

• Using MEMORY USAGE to get overall memory info
• Typing invalid commands like SHOW MEMORY
• Confusing command names

3. Given this Redis INFO memory output snippet:

used_memory:1048576
used_memory_peak:2097152
mem_fragmentation_ratio:1.5

What does the mem_fragmentation_ratio value of 1.5 indicate?

medium

A. Redis memory is fragmented, using 1.5 times more memory than needed

B. Redis is using 50% more memory than allocated

C. Redis has 1.5 million keys stored

D. Redis memory usage is optimal

Solution

1. Step 1: Understand mem_fragmentation_ratio

   This ratio shows how much extra memory Redis uses due to fragmentation.

2. Step 2: Interpret value 1.5

   A value of 1.5 means Redis uses 1.5 times the memory it actually needs, indicating fragmentation.

3. Final Answer:

   Redis memory is fragmented, using 1.5 times more memory than needed -> Option A

4. Quick Check:

   Fragmentation ratio > 1 means extra memory used [OK]

Hint: Fragmentation ratio >1 means extra memory used [OK]

Common Mistakes:

• Thinking ratio shows number of keys
• Assuming ratio below 1 means fragmentation
• Confusing fragmentation with peak memory

4. You run INFO memory and see used_memory_peak is lower than expected despite high used_memory. What is the likely cause?

medium

A. The command syntax is incorrect

B. Redis is leaking memory and reporting wrong values

C. Redis memory peak resets after restart, but current usage is higher

D. Redis has no keys stored

Solution

1. Step 1: Understand used_memory_peak

   This shows the highest memory Redis used since last restart.

2. Step 2: Compare with used_memory

   If current used_memory is higher, it means memory usage increased after peak was reset (likely after restart).

3. Final Answer:

   Redis memory peak resets after restart, but current usage is higher -> Option C

4. Quick Check:

   Peak resets on restart, current usage can be higher [OK]

Hint: Peak resets on restart; current can exceed peak after restart [OK]

Common Mistakes:

• Assuming memory leak without evidence
• Thinking command syntax is wrong
• Believing no keys means memory usage

5. You notice your Redis server's mem_fragmentation_ratio is consistently above 2.0. What is the best action to reduce memory fragmentation?

hard

A. Run MEMORY PURGE command to defragment memory

B. Increase the number of keys stored

C. Restart Redis to reset memory fragmentation

D. Delete all keys to free memory

Solution

1. Step 1: Understand high fragmentation impact

   A ratio above 2 means Redis uses twice the memory needed, hurting performance.

2. Step 2: Identify Redis command to reduce fragmentation

   MEMORY PURGE triggers allocator to release unused memory and reduce fragmentation.

3. Step 3: Evaluate other options

   Restarting helps temporarily but is disruptive. Increasing keys worsens memory. Deleting all keys is drastic and unnecessary.

4. Final Answer:

   Run MEMORY PURGE command to defragment memory -> Option A

5. Quick Check:

   Use MEMORY PURGE to reduce fragmentation [OK]

Hint: Use MEMORY PURGE to reduce fragmentation without restart [OK]

Common Mistakes:

• Restarting Redis as first fix
• Adding more keys to fix memory
• Deleting all keys unnecessarily