Placement groups in AWS EC2 are a feature that allows you to control the placement of instances within the AWS infrastructure.

  1. Definition: Placement groups are logical groupings of instances within the same Availability Zone.
  2. Purpose: They are designed to influence the physical placement of instances to meet specific requirements, such as low-latency communication or high availability.
  3. Types: AWS offers three types of placement groups: Cluster, Spread, and Partition.
  4. Cluster Placement Group:
    • Instances are placed in close proximity to each other.
    • Designed for low-latency, high-throughput applications.
    • Instances are typically located on the same rack.
  5. Spread Placement Group:
    • Instances are placed on distinct hardware (racks) to reduce the risk of correlated failures.
    • Suitable for critical applications that require high availability.
  6. Partition Placement Group:
    • Useful for large distributed and replicated workloads.
    • Instances are spread across logical partitions to ensure resource isolation.
  7. Limitations: Not all instance types are supported in all types of placement groups. You should consult AWS documentation for specific instance type and placement group compatibility.
  8. Cluster Size: Cluster placement groups can host a maximum of 7 instances per Availability Zone.
  9. Spread Placement Size: Spread placement groups can span multiple racks but can only host a maximum of 7 instances per rack.
  10. Partition Placement Size: Partition placement groups can support thousands of instances in multiple partitions.
  11. Instance Affinity: Instances in a placement group have affinity with each other, meaning they are more likely to be co-located.
  12. Instance Anti-Affinity: Spread and Partition placement groups ensure instances are placed with anti-affinity, reducing the risk of co-location.
  13. Network Performance: Cluster placement groups offer the best network performance as instances are physically close.
  14. Instance Restart: In case of hardware failure, AWS will attempt to restart the instance in the same placement group to maintain affinity.
  15. Instance Move: You can’t move an existing instance into a different placement group; you have to create a new instance in the desired group.
  16. Availability Zones: Placement groups are tied to specific Availability Zones, so instances in different groups won’t span zones.
  17. Use Cases for Cluster Groups: HPC (High-Performance Computing), Big Data, and applications that require low-latency communication.
  18. Use Cases for Spread Groups: Critical applications, small to medium-sized databases, and services requiring high availability.
  19. Use Cases for Partition Groups: Large distributed databases, data warehouses, and applications that need resource isolation.
  20. Elastic Load Balancing: You can use Elastic Load Balancing with instances in a placement group to distribute traffic across instances.
  21. Auto Scaling: You can configure Auto Scaling to launch instances in a specific placement group.
  22. Data Transfer Costs: Data transfer between instances in the same placement group is usually free or at reduced cost.
  23. Cost Considerations: While placement groups are free to create, they can influence the underlying instance placement, which might affect your overall cost.
  24. Instance Types: Different instance types may have varying levels of support for placement groups.
  25. Changing Group Types: You cannot change the type of a placement group once it’s created.
  26. Spot Instances: Spot Instances can be part of placement groups, but you should be mindful of potential interruptions.
  27. Dedicated Hosts: You can use Dedicated Hosts to have even more control over placement, but this comes at a higher cost.
  28. AWS CLI and SDK: You can create and manage placement groups using the AWS CLI or SDKs.
  29. Monitoring and Metrics: You can monitor placement groups using AWS CloudWatch metrics to track their performance.
  30. Backup and Recovery: Consider the implications of placement groups on backup and recovery strategies for your applications.

Placement groups can significantly impact the performance and reliability of your EC2 instances, so it’s important to choose the right type and configure them carefully based on your application’s requirements and architecture.

By Abhishek K.

Author is a Architect by profession. This blog is to share his experience and give back to the community what he learned throughout his career.