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.

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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.