4.19 EC2 Placement Groups - Exam Must Know

Placement Groups

A placement group is a logical grouping of instances within a single AZ. Using placement groups enables applications to participate in a low-latentcy, 10Gbps network. Placement groups are recommended for applications that benefit from low network latency, high network throughput, or both.

Essentially, in real life way you would use placement groups of things like grid computing, Hadoop cluster, or if you use Cassandra as your back-end database you will need high network throughput or low latency between your different Cassandra nodes.

Placement group is used primarily for very low network latency and very high network throughput. The whole point of the placement group is this 10 Gbps network that you get.

You can launch instances in a placement group, which determines how instances are placed on underlying hardware. When you create a placement group, you specify one of the following strategies for the group:

• Cluster — clusters instances into a low-latency group in a single AZ. To provide the lowest latency, and the highest packet-per-second network performance for your placement group, choose an instance type that supports enhanced networking. We recommend that you launch the number of instances that you need in the placement group in a single launch request and that you use the same instance type for all instances in the placement group. Restarting/rebooting the instances may migrate them to hardware that has capacity for all the requested instances if you receive a capacity error when launching an instance in a placement group that already has running instances.

• Spread — spreads instances across underlying hardware. A spread placement group is a group of instances that are each placed on distinct underlying hardware. Spread placement groups are 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 underlying hardware. A spread placement group can span multiple AZs, and you can have a maximum of seven running instances per Availability Zone per group.

There is no charge for creating a placement group.

Terms

• Enhanced networking: Enhanced networking uses single root I/O virtualization (SR-IOV) to provide high-performance networking capabilities on some supported instance types. SR-IOV is a method of device virtualization that provides higher I/O performance and lower CPU utilization when compared to traditional virtualized network interfaces. There is no additional charge for using enhanced networking.

Exam Tips

• A placement group cannot span multiple AZs, because it need to guarantee the low latency and high throughput. This may cause the Single Point Failure.

• The name you specify for a placement group must be unique within your AWS account.

• Only certain types of instances can be launched in a placement group (Compute Optimized, GPU, Memory Optimized, Storage Optimized), you cannot use a t2-micro instance in your placement group.

• AWS recommend homogenous instances within placement groups. Homogenous instances can be simply seen as the instances which have same size and same family, so you don't want to have a mixture of different instance sizes from different families within a placement group.

• You cannot merge placement groups.

• You also cannot add an existing instance into your placement group. You can create an AMI from your existing instance, and then launch a new instance from the AMI into a placement group.