How to Fix Redis OOM Command Not Allowed Error (Maxmemory Policy Setup)

✨ Quick Answer

The OOM command not allowed when used memory > 'maxmemory' error occurs when a Redis instance reaches its configured memory ceiling (maxmemory) while operating under the default noeviction policy. Under these conditions, Redis rejects all write operations (SET, HSET, LPUSH, SADD) with an Out-of-Memory exception to protect existing data from unmanaged deletion.

The OOM command not allowed when used memory > 'maxmemory' error occurs when a Redis instance reaches its configured memory ceiling (maxmemory) while operating under the default noeviction policy. Under these conditions, Redis rejects all write operations (SET, HSET, LPUSH, SADD) with an Out-of-Memory exception to protect existing data from unmanaged deletion.

Quick Diagnostics

Cause

Write commands fail with OOM command not allowed: Memory hit the maxmemory threshold while using the default noeviction policy

Solution

Configure an automatic eviction policy (allkeys-lru / volatile-lru) or increase maxmemory

Cause

The Redis daemon is killed by the Linux Kernel (OOM Killer): redis.conf lacks a explicit maxmemory boundary, exhausting host system RAM

Solution

Set an explicit maxmemory quota (e.g., 70-80% of total host physical memory)

Cause

High memory fragmentation ratio (mem_fragmentation_ratio > 1.5): The Jemalloc allocator holds unreleased pages after bulk key deletion operations

Solution

Enable Redis active defragmentation (activedefrag yes)

Step-by-Step Solution

1. 1

   Step 1: Inspect Redis Memory Usage Statistics

   Connect to your Redis instance using redis-cli and query the memory subsytem status:

   BASH Copy

   # Connect via redis-cli
   redis-cli -h 127.0.0.1 -p 6379
   
   # Query memory statistics
   127.0.0.1:6379> INFO memory
   
   # Key fields to examine:
   # used_memory_human:1.95G
   # maxmemory_human:2.00G
   # maxmemory_policy:noeviction
   

2. 2

   Step 2: Change Eviction Policy (maxmemory-policy) Live

   To restore service immediately without restarting your Redis cluster or process, update the eviction policy at runtime:

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   # Enable Least Recently Used (LRU) eviction across all keys
   127.0.0.1:6379> CONFIG SET maxmemory-policy allkeys-lru
   
   # Optional: Dynamically increase maxmemory if free host memory is available
   127.0.0.1:6379> CONFIG SET maxmemory 3gb
   

   Key Redis eviction policies explained:

   • allkeys-lru: Evicts the least recently used keys out of all keys. Recommended for web caching layers.
   • volatile-lru: Evicts least recently used keys among those with an explicit TTL set.
   • allkeys-lfu: Evicts the least frequently used keys across the dataset.
   • noeviction: Rejects all write commands with an OOM error (default setting).
3. 3

   Step 3: Persist Changes in Configuration File (redis.conf)

   To ensure settings persist across server reboots or container restarts, update your primary redis.conf:

   INI Copy

   # Path: /etc/redis/redis.conf or custom redis.conf
   
   # Maximum RAM allocated to Redis (e.g., 2 Gigabytes)
   maxmemory 2048mb
   
   # Production eviction policy for cache workloads
   maxmemory-policy allkeys-lru
   
   # Enable background active defragmentation
   activedefrag yes
   

   Apply and verify the system service status:

   BASH Copy

   sudo systemctl restart redis-server