Slow log for performance analysis in Redis

Introduction

The slow log helps find commands that take a long time to run in Redis. This helps you see what makes your database slow.

You want to find which commands are slowing down your Redis server.

You need to improve the speed of your Redis database.

You want to check if some commands take longer than expected.

You want to monitor Redis performance during heavy use.

You want to debug performance issues in your Redis setup.

Syntax

Redis

SLOWLOG GET [count]
SLOWLOG LEN
SLOWLOG RESET

SLOWLOG GET [count] shows the slow commands, optionally limited by count.

SLOWLOG LEN shows how many slow log entries are stored.

SLOWLOG RESET clears all slow log entries.

Examples

Shows the 5 most recent slow commands recorded by Redis.

Redis

SLOWLOG GET 5

Shows the total number of slow log entries currently stored.

Redis

SLOWLOG LEN

Clears all slow log entries to start fresh.

Redis

SLOWLOG RESET

Sample Program

This example clears the slow log, sets the threshold to 10 milliseconds, runs a fast command (PING), then a slow command (DEBUG SLEEP 0.02), and finally gets the last 2 slow log entries.

Redis

SLOWLOG RESET
CONFIG SET slowlog-log-slower-than 10000
PING
DEBUG SLEEP 0.02
SLOWLOG GET 2

Important Notes

The slow log records commands that take longer than the configured threshold (in microseconds).

By default, the slow log threshold is 10,000 microseconds (10 ms).

Use CONFIG SET slowlog-log-slower-than 0 to log all commands.

Summary

The slow log helps find slow commands in Redis.

You can view, count, and clear slow log entries.

Adjust the threshold to control what is logged.

Practice

1. What is the primary purpose of the Redis slow log?

easy

A. To clear the Redis cache automatically

B. To store all commands executed in Redis

C. To record commands that take longer than a set time to execute

D. To backup Redis data periodically

Solution

1. Step 1: Understand what slow log tracks

   The slow log records commands that exceed a certain execution time threshold.

2. Step 2: Differentiate from other Redis features

   It does not store all commands, clear cache, or backup data; it only logs slow commands.

3. Final Answer:

   To record commands that take longer than a set time to execute -> Option C

4. Quick Check:

   Slow log = records slow commands [OK]

Hint: Slow log tracks only slow commands, not all commands [OK]

Common Mistakes:

• Thinking slow log stores all commands
• Confusing slow log with cache clearing
• Assuming slow log is for backups

2. Which Redis command correctly retrieves the slow log entries?

easy

A. SLOWLOG LIST

B. SLOWLOG FETCH

C. SLOWLOG SHOW

D. SLOWLOG GET

Solution

1. Step 1: Recall the Redis slow log retrieval command

   The official command to get slow log entries is SLOWLOG GET.

2. Step 2: Verify other options

   Commands like SLOWLOG FETCH, SLOWLOG SHOW, and SLOWLOG LIST do not exist in Redis.

3. Final Answer:

   SLOWLOG GET -> Option D

4. Quick Check:

   Retrieve slow log = SLOWLOG GET [OK]

Hint: Use SLOWLOG GET to view slow log entries [OK]

Common Mistakes:

• Using non-existent commands like SLOWLOG FETCH
• Confusing with LIST or SHOW commands
• Misspelling the command

3. Given the following Redis commands executed in order:

SLOWLOG RESET
SLOWLOG LEN
SLOWLOG GET 2

What will be the output of SLOWLOG LEN immediately after the reset?

medium

A. 0

B. 2

C. An error message

D. The last 2 slow log entries

Solution

1. Step 1: Understand SLOWLOG RESET effect

   The SLOWLOG RESET command clears all slow log entries, so the log becomes empty.

2. Step 2: Check SLOWLOG LEN after reset

   Since the log is empty, SLOWLOG LEN returns 0, indicating no entries.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Reset clears log, length = 0 [OK]

Hint: Reset clears log; length after reset is zero [OK]

Common Mistakes:

• Expecting old entries after reset
• Confusing SLOWLOG LEN with SLOWLOG GET output
• Thinking reset causes error

4. You run SLOWLOG GET but receive an empty list even though you expect slow commands. What is the most likely cause?

medium

A. The slow log threshold is set too high, so no commands qualify

B. The Redis server is down

C. You need to restart Redis to enable slow log

D. The slow log is full and cannot record more entries

Solution

1. Step 1: Understand slow log threshold role

   Redis only logs commands slower than the configured threshold (default 10ms). If threshold is too high, no commands get logged.

2. Step 2: Eliminate other causes

   Redis server down would cause connection errors, not empty logs. Restart is not needed to enable slow log. Slow log size is limited but overwrites old entries, so it won't be empty due to fullness.

3. Final Answer:

   The slow log threshold is set too high, so no commands qualify -> Option A

4. Quick Check:

   High threshold = no slow commands logged [OK]

Hint: Check slow log threshold if no entries appear [OK]

Common Mistakes:

• Assuming Redis must restart to enable slow log
• Thinking slow log fills up and stops logging
• Confusing server down with empty slow log

5. You want to analyze slow commands that took longer than 50 milliseconds and keep only the last 5 entries. Which sequence of Redis commands achieves this?

hard

A. CONFIG SET slowlog-log-slower-than 50
SLOWLOG RESET
SLOWLOG GET 5

B. CONFIG SET slowlog-max-len 5
CONFIG SET slowlog-log-slower-than 50000
SLOWLOG RESET
SLOWLOG GET 5

C. CONFIG SET slowlog-log-slower-than 50000
SLOWLOG RESET
SLOWLOG GET 5

D. CONFIG SET slowlog-max-len 5
CONFIG SET slowlog-log-slower-than 50
SLOWLOG RESET
SLOWLOG GET 5

Solution

1. Step 1: Set threshold to 50 milliseconds

   Redis uses microseconds, so 50 ms = 50000 μs. Use CONFIG SET slowlog-log-slower-than 50000. A and C correct; B and D use 50 μs (too low).

2. Step 2: Limit slow log length to 5 entries

   Use CONFIG SET slowlog-max-len 5. Only A and D do this; B and C do not.

3. Step 3: Reset slow log and get last 5 entries

   SLOWLOG RESET clears old entries; SLOWLOG GET 5 retrieves last 5. All options include this.

4. Step 4: Choose correct option

   Only A sets both max-len to 5 and threshold to 50000 μs (50 ms).

5. Final Answer:

   CONFIG SET slowlog-max-len 5 CONFIG SET slowlog-log-slower-than 50000 SLOWLOG RESET SLOWLOG GET 5 -> Option B

6. Quick Check:

   Threshold 50ms = 50000μs, max len 5, reset, get 5 [OK]

Hint: Set threshold in microseconds and max length before reset [OK]

Common Mistakes:

• Confusing milliseconds with microseconds in threshold
• Not setting max length to limit entries
• Forgetting to reset slow log before analysis