When you launch a new EC2 instance, the EC2 service attempts to place the instance in such a way that all of your instances are spread out across underlying hardware to minimize correlated failures. You can use placement groups to influence the placement of a group of interdependent instances to meet the needs of your workload. Depending on the type of workload, you can create a placement group using one of the following placement strategies:
A Cluster placement group packs instances close together inside an Availability Zone. This strategy enables workloads to achieve the low-latency network performance necessary for tightly-coupled node-to-node communication that is typical of HPC applications. Enhanced Networking should be enabled to deliver high throughput and low latency. This delivers 10Gbps rather than standard 5Gbps between instances in the cluster.
Spread placement groups provides isolation between instances. It places a small group of instances across distinct underlying hardware to reduce correlated failure. It is limited to 7 instances per AZ.
This approach is required when high availability is required for a small number of instances.
Partition placement groups spread your instances across logical partitions such that groups of instances in one partition do not share the underlying hardware with groups of instances in different partitions. Or put another way, each partition has its own rack which does not share infrastructure with other racks. There is a maximum of 7 partitions per AZ but you can have much more instances per partition
This strategy is typically used by large distributed and replicated workloads, such as Hadoop, Cassandra, and Kafka.