Spread placement groups are a strategic EC2 instance placement option in AWS designed to maximize availability and reduce correlated failures for critical workloads. When you launch instances in a spread placement group, AWS ensures each instance is placed on distinct underlying hardware, meaning s…Spread placement groups are a strategic EC2 instance placement option in AWS designed to maximize availability and reduce correlated failures for critical workloads. When you launch instances in a spread placement group, AWS ensures each instance is placed on distinct underlying hardware, meaning separate physical racks with independent power sources and network connectivity.
Key characteristics of spread placement groups include a limit of seven running instances per Availability Zone per group. This constraint exists because AWS guarantees physical separation, and data center rack availability is finite. However, you can span spread placement groups across multiple Availability Zones within a region to increase your total instance count.
From a cost optimization perspective, spread placement groups themselves incur no additional charges. The benefit lies in architectural resilience - by distributing instances across isolated hardware, you reduce the risk of simultaneous failures affecting multiple instances. This can lower costs associated with downtime and recovery operations.
Performance considerations include understanding that spread placement groups prioritize fault isolation over network performance. Unlike cluster placement groups optimized for low-latency communication, spread groups may have instances on physically distant racks, potentially introducing slightly higher network latency between grouped instances.
Ideal use cases include small numbers of critical instances where hardware failure correlation must be minimized, such as primary and standby database servers, or application servers requiring high availability. For SysOps administrators, monitoring placement group compliance through AWS Config rules ensures instances maintain their intended distribution.
When creating spread placement groups via the AWS Console, CLI, or CloudFormation, specify the strategy as 'spread' during group creation. Instances can only be launched into a spread placement group if sufficient distinct hardware is available. Launch failures occur when the placement constraint cannot be satisfied, requiring you to try again later or use a different Availability Zone.
Spread Placement Groups - Complete Guide for AWS SysOps Administrator Associate
What are Spread Placement Groups?
Spread placement groups are a logical grouping of EC2 instances that ensures each instance is placed on distinct underlying hardware (separate racks). This strategy minimizes the risk of simultaneous failures affecting multiple instances in your application.
Why are Spread Placement Groups Important?
Spread placement groups are critical for:
• High Availability: By placing instances on separate physical hardware, you reduce the risk of correlated hardware failures • Fault Isolation: Each instance runs on its own rack with independent network and power sources • Critical Workloads: Ideal for applications where instance failure isolation is paramount • Small Number of Critical Instances: Perfect for scenarios where you have a limited number of instances that must remain operational
How Spread Placement Groups Work
• Each instance is placed on a distinct rack within an Availability Zone • Each rack has its own network and power source • You can have a maximum of 7 running instances per Availability Zone per spread placement group • Spread placement groups can span multiple Availability Zones within the same region • If you need more than 7 instances in an AZ, you must create additional spread placement groups
Key Characteristics:
• Rack-level isolation: Instances are separated at the hardware rack level • Multi-AZ support: Can span across multiple Availability Zones • 7 instance limit per AZ: This is a hard limit per placement group • No additional charges: Using placement groups does not incur extra costs
Use Cases for Spread Placement Groups
• Applications requiring high availability with a small number of critical instances • Databases with primary and replica instances that must survive hardware failures • Applications where each host must be isolated from failures on other hosts • Mission-critical applications that cannot tolerate simultaneous instance failures
Limitations to Remember
• Maximum of 7 running instances per AZ per spread placement group • Cannot merge placement groups • An instance can only be launched in one placement group at a time • Dedicated Instances can be launched into spread placement groups, but Dedicated Hosts cannot • Not all instance types support placement groups
Exam Tips: Answering Questions on Spread Placement Groups
• Remember the 7 instance limit: If a question mentions needing more than 7 instances in a single AZ, spread placement groups alone will not suffice
• Understand the difference from Cluster placement groups: Cluster groups are for low latency and high throughput (instances close together), while Spread groups are for fault tolerance (instances separated)
• Rack-level isolation is key: When questions mention hardware failure isolation or rack-level separation, think Spread placement groups
• Multi-AZ capability: Spread groups can span AZs, which is useful for disaster recovery scenarios
• Small, critical workloads: When the scenario describes a small number of critical instances requiring maximum isolation, Spread is the answer
• Watch for insufficient capacity errors: If launching an instance fails due to capacity, you may need to stop and start existing instances or try a different instance type
• Partition vs Spread: If you need more than 7 instances with hardware isolation, consider Partition placement groups instead, which allow larger deployments with logical partitions
• Look for keywords: Terms like hardware failure isolation, separate racks, distinct hardware, and critical instances often point to Spread placement groups