This ratio is an indication of memory mapping efficiency:
  — Value over 1.0 indicate that memory fragmentation is very likely. Consider restarting the Redis server so the operating system can recover fragmented memory, especially with a ratio over 1.5.
  — Value under 1.0 indicate that Redis likely has insufficient memory available. Consider optimizing memory usage or adding more RAM.

Note: If your peak memory usage is much higher than your current memory usage, the memory fragmentation ratio may be unreliable.

https://redis.io/topics/memory-optimization