SysOps Administrator: Understanding Placement Groups

What are Placement Groups?

Placement groups are an AWS feature that allows you to have control over the placement of EC2 instances within the AWS infrastructure. They are used to define how instances are placed in relation to each other, without direct interaction with the hardware.

Types of Placement Groups

There are three strategies available for placement groups:

1. Cluster Placement Group
   • Instances are grouped together in a low-latency hardware setup within a single Availability Zone (AZ).
   • Offers high performance but comes with a high risk of simultaneous failure.
   • Ideal for applications requiring super low latency and high network throughput.
2. Spread Placement Group
   • Instances are spread across different hardware, limiting the risk of simultaneous failure.
   • Each spread placement group can have a maximum of seven EC2 instances per AZ.
   • Suitable for critical applications that need high availability and isolated instance failures.
3. Partition Placement Group
   • Instances are spread across multiple partitions within an AZ.
   • Each partition represents a separate set of hardware racks, providing isolation from failures of other partitions.
   • Supports up to seven partitions per AZ and can span multiple AZs in the same region.
   • Can accommodate hundreds of EC2 instances per group.
   • Ideal for big data applications that are partition aware, such as Hadoop, Cassandra, or Kafka.

Detailed Analysis of Each Placement Group Strategy

Cluster Placement Group

• Pros:
  • Super low latency between instances.
  • High network speeds (up to 10 Gbps).
  • Suitable for big data jobs and applications with low latency and high throughput requirements.
• Cons:
  • If the hardware (rack) fails, all instances may fail simultaneously.
  • Increased risk of infrastructure-related failure.

Spread Placement Group

• Pros:
  • Instances are on different hardware, reducing the risk of simultaneous failure.
  • Improved high availability.
• Cons:
  • Limited to seven instances per AZ, per placement group.
  • Not suitable for very large applications due to the instance limit.

Partition Placement Group

• Pros:
  • Instances are distributed across many hardware racks, each representing a partition.
  • Each partition is isolated from the failures of other partitions.
  • Scales to hundreds of EC2 instances.
  • Access to partition information through the metadata service.
• Cons:
  • Requires applications to be partition aware to effectively distribute data and servers across partitions.

Use Cases for Placement Groups

• Cluster Placement Group: High-performance computing, big data processing, applications needing high network performance.