Redis: OOM command not allowed when used memory > 'maxmemory'

What the message looks like

OOM command not allowed when used memory > 'maxmemory'.

Common causes

• maxmemory-policy noeviction is active, which is the default. Redis evicts nothing and refuses writes instead.
• Keys without a TTL fill memory, for example cache entries that never expire.
• A single large key grows without bound: a list, hash or set used as a queue.
• Fragmentation and replication buffers push used_memory above the limit even though the data is smaller.
• After a restart maxmemory is missing from the config, so Redis grows until the host runs out of memory.

Check

1. Memory, limit and policy
   redis-cli INFO memory | grep -E 'used_memory_human|maxmemory_human|maxmemory_policy'Copy
2. Find the largest keys
   redis-cli --bigkeysCopy
3. Count evicted keys
   redis-cli INFO stats | grep evicted_keysCopy
4. Share of keys without TTL in a sample
   redis-cli --scan --count 1000 | head -n 1000 | while read k; do redis-cli TTL "$k"; done | grep -c '^-1$'Copy

Fix

• Choose the policy on purpose: for a pure cache use allkeys-lru or allkeys-lfu, then Redis makes room itself.
• Keep noeviction only if Redis holds data that must not be lost. Then add memory and set TTLs.
• Set a TTL on every cache key, even the ones that look permanent.
• Split or cap large keys; --bigkeys shows the candidates.
• Keep maxmemory well below host RAM so snapshots and copy-on-write have room.

FAQ

Why does Redis not just drop old keys?

Because maxmemory-policy is noeviction, which is the default.

Do I lose data when I switch to allkeys-lru?

Yes, old keys get evicted. That is fine for a cache, not for queue or session data.