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How It Works
When you launch EC2 instances, AWS decides by default where to place them across its physical infrastructure. Placement Groups let you override that default and specify a placement strategy. AWS offers three types. A Cluster Placement Group packs instances close together on the same physical rack, maximizing network throughput and minimizing latency between instances. A Spread Placement Group distributes instances across distinct hardware racks, reducing the risk that a single hardware failure takes down multiple instances at once. A Partition Placement Group divides instances into logical partitions, each on separate rack infrastructure, which is useful for large distributed systems like Hadoop or Cassandra that need isolation between node groups.
Why It Matters for Cloud Cost
Placement Group type directly affects which instance families and sizes you can run, and by extension, which discount programs apply. Cluster Placement Groups require instance types that support enhanced networking, which often means larger or more expensive instance families. Choosing the wrong placement strategy for a workload can lead to over-provisioning just to meet network performance requirements. Spread Placement Groups cap the number of instances per group per Availability Zone, so teams planning large fleets need to account for this constraint before committing to Reserved Instances or Savings Plans at a particular scale. Poor placement decisions can also drive up inter-node traffic costs if latency-sensitive workloads are spread across zones unnecessarily.
Usage AI’s Autopilot and CoPilot modes manage EC2 commitment coverage through the Usage Flex Savings Plan, saving 40 to 60% versus on-demand pricing with no upfront cost.