Fix: Redis Memory Limit Exceeded

!Symptoms

• Redis returns OOM command not allowed when used memory > maxmemory
• Write operations fail but reads still work
• Application cache errors or increased latency
• Redis logs show 'Can't save in background: fork: Cannot allocate memory'

?Root Causes

• No TTL set on cached keys, causing unbounded growth
• maxmemory-policy set to noeviction (default) so Redis refuses writes
• Large data structures (sorted sets, lists) growing without bounds
• Background save (RDB/AOF) requiring fork doubles memory usage temporarily
• Memory fragmentation ratio is high

#Diagnosis Steps

1. 1Check memory usage: `redis-cli INFO memory`
2. 2Check maxmemory setting: `redis-cli CONFIG GET maxmemory`
3. 3Check eviction policy: `redis-cli CONFIG GET maxmemory-policy`
4. 4Find large keys: `redis-cli --bigkeys`
5. 5Check memory fragmentation ratio in INFO memory output

>Fix

1. 1Set an eviction policy: `redis-cli CONFIG SET maxmemory-policy allkeys-lru`
2. 2Increase maxmemory if the server has available RAM
3. 3Delete unnecessary keys: identify and remove unused data
4. 4Add TTL to keys that should expire: `EXPIRE key seconds`
5. 5Disable background saving if fork is the issue: configure AOF instead of RDB

*Prevention

• Always set maxmemory and an appropriate eviction policy
• Set TTL on all cache keys at write time
• Monitor Redis memory usage with alerts at 80% of maxmemory
• Use Redis Cluster for horizontal scaling when a single node is insufficient
• Regularly audit key patterns and sizes with redis-cli --bigkeys

Related Error Messages