Placement Groups

• 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.

• Can use placement groups to influence the placement of a group of interdependent instances to meet the needs of your workload.

Cluster Placement Groups

• Packs instances close together within only 1 Availability Zone.

  • This means it has not got very high availability as all the instances are in one AZ.

• 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.

• AWS recommends to:

  • Launch all instances at the same time.

  • Use the same instance type for all instances in the placement group.

• Instances within a cluster placement group can use up to 10 Gbps for single-flow traffic.

  • Instances that are not within a cluster placement group can use up to 5 Gbps for single-flow traffic.

Spread Placement Groups

• This strategy strictly places a small group of instances across distinct underlying hardware to reduce correlated failures.

• Recommended for applications that have a small number of critical instances that should be kept separate from each other.

• Launching instances in a spread placement group reduces the risk of simultaneous failures that might occur when instances share the same racks.

• Can span multiple Availability Zones in the same Region.

• You can only have a maximum of 7 running instances per Availability Zone per placement group.

Partition Placement Groups

• This spreads 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.

• It is typically used by large distributed and replicated workloads, such as Hadoop, Cassandra, and Kafka.

• Help reduce the likelihood of correlated hardware failures for your application.

• EC2 divides each group into logical segments called partitions.

  • EC2 ensures that each partition within a placement group has its own set of racks.

  • No two partitions within a placement group share the same racks.

• You can launch instances into a specific partition to have more control.

• You can have partitions in multiple Availability Zones in the same Region.

• You can only have a maximum of 7 partitions per Availability Zone.

  • However, you can have 100s of EC2 Instances in the placement group.

Spread vs Placement Groups

• Spread Placement Groups:

  • Have individual instances all on separate hardware.

  • Recommended for applications that have a small number of critical instances that should be kept separate from each other.

• Partition Placement Groups:

  • Described as large groups of instances where each group is placed on separate hardware.

  • Each partition comprises multiple instances.

  • The instances in a partition do not share racks with the instances in the other partitions, allowing you to contain the impact of a single hardware failure to only the associated partition.