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Placement groups

Concepts

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

Depending on the type of workload, you can create a placement group using one of the following placement strategies:

  • Cluster – packs instances close together inside an Availability Zone. 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.

Cluster placement

  • Partition – 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. This strategy is typically used by large distributed and replicated workloads, such as Hadoop, Cassandra, and Kafka.

Partition placement

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

Spread placement

There is no charge for creating a placement group.

Exam tips

  • Clustered placement group
    • Low Network Latency
    • High network Throughput
    • Can not span over multiple Availability Zones
    • AWS recommends homogenous instances in cluster placement group
  • Spread Placement Group
    • Individual Critical EC2 Instances
    • Span over multiple Availability Zones
  • Partitioned Placement Group
    • Multiple EC2 instances like in distributed workloads HDFS, HBase & Cassandra
    • Can span over multiple Availability Zones
  • Placement Group name is unique per AWS account
  • Only some instance types can be launched in placement groups
    • Compute optimize
    • GPU
    • Memory optimize
    • Storage optimize
  • You can't merge placement groups.
  • You can move instances from one Placement Group to other but the instance needs to be in stopped state. You can do it with the CLI, API but not in the Console.