Memory usage analysis (INFO memory) in Redis

Introduction

Memory usage analysis helps you understand how much memory your Redis server is using. This keeps your database fast and stable.

You want to check if your Redis server has enough memory for your data.

You notice Redis is slow and want to see if memory is the cause.

You want to plan for scaling your Redis server by knowing current memory use.

You want to find out how much memory different parts of Redis are using.

You want to monitor memory to avoid crashes due to running out of memory.

Syntax

Redis

INFO memory

This command returns detailed memory usage information from Redis.

You run it in the Redis command line or through a Redis client.

Examples

Shows all memory-related information about the Redis server.

Redis

INFO memory

Sample Program

This command returns memory stats like total used memory, peak memory, fragmentation ratio, and allocator info.

Redis

INFO memory

Important Notes

The 'used_memory' value shows the total bytes Redis is using.

The 'used_memory_rss' shows how much memory the operating system reports Redis is using.

A high 'mem_fragmentation_ratio' means Redis memory is fragmented and may waste RAM.

Summary

Use 'INFO memory' to check Redis memory usage anytime.

It helps you keep Redis fast by monitoring memory use.

Look at 'used_memory', 'used_memory_peak', and 'mem_fragmentation_ratio' for key info.

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