Slow log for performance analysis in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the Redis slow log?

The Redis slow log records commands that take longer than a specified time to execute, helping identify performance bottlenecks.

intermediate

How do you configure the minimum execution time for commands to be logged in Redis slow log?

Use the configuration parameter slowlog-log-slower-than to set the minimum execution time in microseconds for commands to be logged.

beginner

Which Redis command retrieves the slow log entries?

The SLOWLOG GET [count] command retrieves the latest slow log entries, where count is optional and limits the number of entries returned.

intermediate

What information does each slow log entry contain?

Each entry includes the unique entry ID, timestamp, execution time in microseconds, and the command arguments executed.

beginner

How can you clear all entries from the Redis slow log?

Use the command SLOWLOG RESET to clear all slow log entries.

What does the Redis slow log help you find?

A Commands that take a long time to execute

B Commands that failed to execute

C Commands that use too much memory

D Commands that are frequently used

The slow log records commands that take longer than a set time to execute, helping find slow commands.

Which command shows the slow log entries in Redis?

A SLOWLOG LIST

B SLOWLOG GET

C SLOWLOG SHOW

D SLOWLOG DISPLAY

The correct command to retrieve slow log entries is SLOWLOG GET.

What unit is used to measure command execution time in the Redis slow log?

A Nanoseconds

B Seconds

C Milliseconds

D Microseconds

Execution time in the slow log is measured in microseconds.

How do you clear all slow log entries in Redis?

A SLOWLOG DELETE

B SLOWLOG CLEAR

C SLOWLOG RESET

D SLOWLOG REMOVE

The command SLOWLOG RESET clears all slow log entries.

Which configuration sets the minimum time for logging slow commands?

A slowlog-log-slower-than

B slowlog-min-time

C slowlog-threshold

D slowlog-time-limit

The parameter slowlog-log-slower-than sets the minimum execution time in microseconds for commands to be logged.

Explain how to use the Redis slow log to find and analyze slow commands.

Think about commands that take too long and how Redis helps you spot them.

Describe the steps to reset the Redis slow log and why you might want to do it.

Consider cleaning up old data to focus on new performance issues.

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