The default policy
noeviction, which refuses writes at the limit while reads keep working
maxmemory: 4gb dataset: 3gb
Redis Eviction Policy Simulator
Give maxmemory, the dataset size and the policy. The result says what happens at the limit and whether the policy can actually do anything.
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Results
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Examples
Real input you can load into the tool above. Each one shows a different thing going wrong, because that is what the tool is for.
volatile with no TTLs
A policy that cannot evict anything, so it behaves exactly like noeviction
maxmemory: 4gb dataset: 3gb maxmemory-policy: volatile-lru keys-with-ttl-percent: 0
Common mistakes
These are the ones that fail silently. The config is accepted, nothing raises an error, and the consequence arrives later.
Leaving the default on a cache
noeviction refuses writes at the limit. For a cache that is an outage, and it arrives by omission rather than by decision.
Instead:allkeys-lru or allkeys-lfu for a cache.
Choosing volatile- without TTLs on the keys
Only keys with a TTL are eligible. With none, nothing can be evicted and writes fail exactly as with noeviction.
Instead:Confirm keys actually get a TTL, or use the allkeys- variant.
Expecting LRU to be exact
Redis samples maxmemory-samples keys and evicts the best of the sample, not the true least-recently-used key.
Instead:Raise the sample size for accuracy, at a CPU cost. LFU is usually better where a small set is hot.
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