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

Read Time: 4 min Updated: 2026-07-27

Quick Diagnostics

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

Symptom Root Cause Solution
Write commands fail with OOM command not allowed Memory hit the maxmemory threshold while using the default noeviction policy Configure an automatic eviction policy (allkeys-lru / volatile-lru) or increase maxmemory
The Redis daemon is killed by the Linux Kernel (OOM Killer) redis.conf lacks a explicit maxmemory boundary, exhausting host system RAM Set an explicit maxmemory quota (e.g., 70-80% of total host physical memory)
High memory fragmentation ratio (mem_fragmentation_ratio > 1.5) The Jemalloc allocator holds unreleased pages after bulk key deletion operations Enable Redis active defragmentation (activedefrag yes)

🚀 Step-by-Step Solution

Step 1: Inspect Redis Memory Usage Statistics

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

BASH 
# 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

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:

BASH 
# 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).

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 
# 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 
sudo systemctl restart redis-server

Prevention Advice

Recommended security practices:

  • Enforce Key TTLs: Always specify Time-To-Live expiration intervals when writing cache entries from application code (SET key value EX 3600) to enable natural key garbage collection.
  • Scan for Big Keys: Routinely scan for bloated data structures using built-in analysis tools: redis-cli --bigkeys or redis-cli --memkeys.
  • Configure Memory Alerts: Set up monitoring alerts in Prometheus or Datadog to notify team members when used_memory exceeds 80% of configured maxmemory.