https://github.com/awsdocs/amazon-ec2-user-guide/blob/master/doc_source/placement-groups.md
You can launch or start instances in a placement group, which determines how instances are placed on underlying hardware. When you create a placement group, you specify one of the following strategies for the group:
- Cluster – clusters instances into a low-latency group in a single Availability Zone
- Partition – spreads instances across logical partitions, ensuring that instances in one partition do not share underlying hardware with instances in other partitions
- Spread – spreads instances across underlying hardware
There is no charge for creating a placement group.
Examp Tips:
- A clustered placement group can't span multiple AZ.
- A spread placement group can.
- The name must be unique within your AWS account.
- Only certain types of instances can be launched in a placement group (Compute Optimized, GPU, Memory Optimized, Storage Optimized).
- AWS recommend homogenous instances within placement groups.
- You can't merge placement groups.
- You can't move an existing instances into a placement group. You can create an AMI from your existing instance, and then launch a new instance from the AMI into a placement group.