Amazon ElastiCache Performance and Memory Economics

With your ElastiCache cluster designed for high availability and global reach through Multi-AZ failover and Global Datastore, the next operational challenge shifts from resilience to runtime efficiency. A cache that survives failures but delivers poor hit rates or costs more than the database it protects is not doing its job. Memory in ElastiCache functions simultaneously as a latency lever and a cost lever. Every byte of DRAM you provision determines how much data stays close to your application at sub-millisecond speed, and every byte you waste inflates your monthly bill without improving throughput.

This lesson walks through five performance and cost levers that govern how well an ElastiCache deployment actually performs under real workloads. You will learn how eviction policies determine whether a cache degrades gracefully or fails abruptly when memory fills, how persistence mechanisms in Valkey and Redis OSS introduce write-latency trade-offs, how connection storms and hot-key concentration create bottlenecks even when aggregate capacity looks healthy, and how data tiering on r6gd-based nodes changes the price-per-gigabyte equation by blending DRAM with local NVMe SSD.

The maxmemory-policyA configuration directive that tells the ElastiCache engine which keys to remove when the node's memory limit is reached, directly controlling cache behavior under pressure. sits at the center of eviction behavior. Choosing the wrong policy for your workload means the cache either throws errors at your application or evicts the data your users need most. Four CloudWatch metrics deserve constant attention throughout this discussion. CacheHitRate tells you how often the cache serves a request without falling back to the origin database. EngineCPUUtilization reveals per-node processing pressure. CurrConnections tracks how many open TCP sessions the engine is managing. Evictions counts how many keys the engine has forcibly removed to free memory. Together, these signals form the operational dashboard for every tuning decision covered below.

Eviction policies and cache hit rate

When an ElastiCache node reaches its memory ceiling, the engine must decide what to do with incoming writes. The maxmemory-policy setting controls that decision, and the choice has a direct impact on whether your application experiences a graceful performance dip or an outright failure.

How each policy behaves under pressure