EC2 Placement Groups:

• A placement group is a cluster of instances within the same availability zone.
• Used for applications that require an extremely low latency network between them.
• AWS attempts to place all the instances as close as physically possible in the data center to reduce latency.
• Instances within a placement group have a low-latency, 10 Gbps network connection between them
• Instances that are in the placement group need to have enhanced networking in order to maximize placement group.

Troubleshooting Placement Groups:

• If an instance in a placement group is stopped, once it is started again it will continue to be member of the placement group.
• It is suggested to launch all the required instances within a placement group in a single request, and that the same instance type is used for all instances within the placement group.
• It is possible, if more instances are added at a later time to the placement group OR if a placement group instance is stopped and started again, to receive an "insufficient capacity error".
  • Resolve the capacity error by stopping all instances in the member group and attempting to start them again.

Important facts about Placement Groups:

• Instances not originally launched/created in the placement group can NOT be moved into the placement group.

• Placement groups can NOT be merged together.

• A placement group can NOT span across multiple Availability Zones.

• Placement group names must be unique within your own AWS account.

• Placement groups can be "connected" through low latency connections, but the speed between those two placement groups will be limited to the latency of the connection between two placement groups themselves, and that will not be as fast as the connections inside of the actual placement group themselves.

• Instances must have 10 gigabits network speeds in order to take advantage of placement groups (proper instance type).