Redis maxmemory-policy: Configure Eviction Strategies for Production

Quick Answer

• What it does: maxmemory-policy controls how Redis evicts keys when memory usage hits the maxmemory limit. Choose allkeys-lru for most cache workloads, volatile-ttl for mixed cache/persistent data, or noeviction to reject writes instead of evicting.
• First check: Run redis-cli INFO stats and inspect used_memory, maxmemory, evicted_keys, and keyspace_hits/keyspace_misses to understand current memory pressure and hit rates before changing policy.
• Minimal fix: Set maxmemory 256mb and maxmemory-policy allkeys-lru in redis.conf, or apply at runtime with redis-cli CONFIG SET maxmemory-policy allkeys-lru.
• Version boundary: All policies listed here are available in Redis 6.0+. The allkeys-lrm policy (LFU with probabilistic decay) is newer; verify availability with redis-cli CONFIG GET maxmemory-policy before relying on it.
• Critical gotcha: With volatile-* policies, if no keys have TTL set, Redis behaves like noeviction and returns OOM errors on writes.

What Problem It Solves

Redis stores data in memory by design. Without a memory cap, a runaway workload can exhaust RAM and crash the process or the host. maxmemory sets the ceiling; maxmemory-policy decides what happens when that ceiling is reached.

The policy answers one question: which keys get evicted when memory is full?

This matters for:

• Cache layers: Hot data should survive eviction; cold data should go first.
• Mixed workloads: You may store both cache data (expiring) and business data (persistent) in the same instance. volatile-* policies protect non-expiring keys.
• Write-heavy systems: If you cannot afford data loss, noeviction rejects writes with an OOM error instead of silently dropping keys.

Redis offers eight eviction policies, split into two families:

Plus noeviction, which evicts nothing and returns an error on writes.

When This Error or Setup Appears

You typically encounter maxmemory-policy in three situations:

1. OOM errors: OOM command not allowed when used memory > 'maxmemory' appears when the current policy is noeviction (the default) and memory is exhausted. Writes fail until memory frees up.
2. Performance degradation: evicted_keys climbs, keyspace_hits drops, and cache effectiveness collapses because the wrong keys are being evicted.
3. Capacity planning: You are deploying a new Redis instance and need to decide how it behaves under memory pressure before it happens.

The configuration is relevant for any Redis deployment—standalone, Sentinel-managed, or Cluster—where memory is finite and eviction behavior matters.

Minimal Working Configuration

Add these lines to redis.conf:

CONF
maxmemory 256mb
maxmemory-policy allkeys-lru
maxmemory-samples 10

Apply at runtime without restart:

BASH
redis-cli CONFIG SET maxmemory 256mb
redis-cli CONFIG SET maxmemory-policy allkeys-lru
redis-cli CONFIG SET maxmemory-samples 10

Verify:

BASH
redis-cli CONFIG GET maxmemory
redis-cli CONFIG GET maxmemory-policy
redis-cli CONFIG GET maxmemory-samples

To make runtime changes persistent, also run:

BASH
redis-cli CONFIG REWRITE

This writes the current configuration back to redis.conf.

Parameters and Environment Variables

Three parameters control eviction behavior:

Policy selection guide

Root Cause Analysis

The OOM error OOM command not allowed when used memory > 'maxmemory' has a specific root cause chain:

1. maxmemory is set to a finite value.
2. maxmemory-policy is noeviction (default) or a volatile-* policy with no expiring keys.
3. Memory usage reaches maxmemory.
4. A write command arrives.
5. Redis cannot evict anything (noeviction) or finds no eligible keys (volatile-* with no TTL keys).
6. Redis rejects the write with an OOM error.

The fix depends on which link in the chain you want to break:

• Raise maxmemory if the limit is too low for the workload.
• Switch to allkeys-* if you want Redis to evict any key when full.
• Set TTLs on keys if you want to keep volatile-* policies.

For low hit rates with high evicted_keys, the root cause is usually a mismatch between the policy and the access pattern:

• allkeys-lru evicts the least recently used key. If your workload has a stable hot set, this works well.
• allkeys-lfu evicts the least frequently used key. If access frequency is more stable than recency (e.g., a product catalog where some items are always popular), LFU outperforms LRU.
• allkeys-random is almost never the right choice unless access is truly uniform.

Common Errors and Fixes

Production Notes and Security Checks

Memory reservation

maxmemory counts only the data itself, not the memory used by replication buffers, AOF buffers, or client output buffers. If you set maxmemory to 100% of available RAM, Redis may enter an eviction feedback loop where it evicts keys to free memory, but the freed memory is immediately consumed by buffers.

Rule of thumb: Set maxmemory to 70–80% of available RAM on dedicated instances. Monitor INFO memory for mem_not_counted_for_evict to see how much overhead exists.

Cluster considerations

In Redis Cluster, maxmemory is configured per node, not cluster-wide. Each node evicts independently. If data is unevenly distributed, one node may hit its limit while others have headroom.

• Set maxmemory per node based on that node's RAM.
• Ensure hash slots distribute evenly to avoid one node becoming a bottleneck.
• Monitor per-node evicted_keys in INFO stats.

Policy safety

• Avoid allkeys-random in production unless you have a specific reason.
• Avoid noeviction for cache workloads—it turns a degraded cache into a failing service.
• Use volatile-* policies only when you consistently set TTLs on cache keys.
• Monitor evicted_keys and keyspace_hits/keyspace_misses continuously. A sudden spike in evicted_keys indicates the policy or maxmemory needs adjustment.

Security

Eviction is not a security boundary. If an attacker can trigger eviction, they can cause denial of service by flooding the cache with keys. Protect Redis with:

• requirepass for authentication
• Binding to trusted interfaces only (bind 127.0.0.1 or internal IPs)
• Disabling CONFIG commands in untrusted environments via rename-command

Comparison With Alternatives

Redis's advantage is flexibility: you can switch policies at runtime without restart, and volatile-* policies let you protect persistent data while evicting cache data. Memcached's single LRU policy is simpler but cannot distinguish between cache and persistent data.

Compared to database-level caching (e.g., MySQL query cache), Redis eviction is more granular and performant, but requires explicit policy management.

FAQ

Q: How do I choose the best eviction policy?

A: Match the policy to your access pattern. If hot data is concentrated (80/20 rule), use allkeys-lru. If access frequency is stable and some keys are always popular, use allkeys-lfu. If access is uniform, allkeys-random may suffice. If you mix cache and persistent data, use volatile-* policies and ensure cache keys have TTLs. Start with allkeys-lru, monitor keyspace_hits and evicted_keys, then adjust.

Q: How does maxmemory-samples affect performance?

A: maxmemory-samples controls how many keys Redis samples before evicting. The default is 5. Higher values produce eviction decisions closer to exact LRU/LFU, but increase CPU usage per eviction. If hit rate is poor, increase to 10 and observe. If CPU is a concern, keep it at 5.

Q: How do I configure maxmemory in Redis Cluster?

A: maxmemory is set per node. Each node's maxmemory should reflect that node's RAM capacity. Eviction runs independently on each node. Ensure data distribution is even across hash slots so no single node hits its limit prematurely. Monitor per-node evicted_keys and used_memory in INFO output.

Q: What happens if I use volatile-lru but no keys have TTL?

A: Redis treats the policy as noeviction. When memory reaches maxmemory, write commands fail with OOM command not allowed when used memory > 'maxmemory'. To fix, either set TTLs on cache keys or switch to an allkeys-* policy.

Q: Can I change the eviction policy without restarting Redis?

A: Yes. Run redis-cli CONFIG SET maxmemory-policy allkeys-lru to change it immediately. Use redis-cli CONFIG REWRITE to persist the change to redis.conf.

Official References

• Redis Eviction Documentation