EC2 - Placement Groups

• Clustered Placement Groups
• A Clustered Placement Group is grouping of instances within a single AZ. 
• Recommended for applications that need low network latency or high network throughput or both. Eg. Cassandra Clusters.
• Only certain instance types can be launched in Clustered Placement Groups.
• Spread Placement Groups
• Spread Placement Group is a grouping of instances such that each instance is launched on a separate underlying hardware.
• Recommended for applications that have a small number of critical instances that should be kept separate from each other.
• Can span across multiple AZs.
• A Clustered Placement Group can not span across multiple AZs while a Spread Placement Group can.
• The name of placement group must be unique within your AWS account.
• Only certain type of instances can be launched within a placement group eg. Compute Optimised, GPU, Memory Optimised, Storage Optimised.
• AWS recommends homogeneous instances within a placement group.
• You can not merge placement groups.
• You can not move an existing instance into a placement group.
• You can create an AMI from existing instance and then launch a new instance using your AMI into a placement group.