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# 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.<br>
* 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.<br>

* 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**.<br>

* **AWS recommends to:**

  * **Launch all instances at the same time**.<br>
  * **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**.<br>

* Recommended **for applications that have a small number of critical instances that should be kept separate from each other**.<br>

* 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.<br>

* 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**.<br>
* It is typically used by **large distributed and replicated workloads**, such as **Hadoop, Cassandra, and Kafka**.<br>
* Help **reduce the likelihood of correlated hardware failures for your application**.<br>
* **EC2 divides each group into logical segments called partitions**.
  * EC2 **ensures that each partition within a placement group has its own set of racks**.<br>
  * **No two partitions within a placement group share the same racks**.<br>
* You can **launch instances into a specific partition to have more control**.<br>
* You **can have partitions in multiple Availability Zones in the same Region**.<br>
* 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**.<br>
  * 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**.<br>
  * **Each partition comprises multiple instances**.<br>
  * 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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