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NOTE

3.3 Redis Slow Log

What Redis Slow Log measures, what it omits, how to configure and inspect it, and how to correlate slow commands with end-to-end latency.

Redis / CacheCreated Updated 1 min readhistorical

This is a historical learning note and may contain outdated or incomplete understanding.

1. What the Slow Log Measures

Redis Slow Log records commands whose server-side execution time exceeds a configured threshold.

It is not the same as end-to-end client latency.

Time can also be spent in:

  • client connection pools;
  • network queues/RTT;
  • server event-loop queueing;
  • response transmission;
  • client-side decoding.

2. Configuration

Important settings include a threshold (slowlog-log-slower-than) and bounded log length (slowlog-max-len). Exact values should be chosen for the workload rather than copied from a generic example.

Commands:

SLOWLOG GET
SLOWLOG LEN
SLOWLOG RESET

3. Common Root Causes

  • big keys;
  • high-complexity collection commands;
  • unbounded scans;
  • long Lua/Functions;
  • huge multi-key commands.

4. Correlate, Don’t Guess

If clients report 100 ms latency but Slow Log shows 1 ms execution, investigate network, queueing, connection pools, CPU scheduling, and output buffers rather than blaming the command itself.

Use Slow Log together with Redis latency monitoring and application tracing.