Proximity Placement Groups

Introduction

• Proximity Placement Groups (PPGs) in Azure
• Key considerations: best practices for usage
• Target audience: users focusing on minimizing latency between services and VM scale sets

Separating Resources in Azure

• Separation for resiliency and isolation from different problem levels:
  • Node level, Rack level, Data Center level, Regional level
• Mechanisms for separation:
  • Availability Sets: Distributes workloads among different fault domains/racks
  • Availability Zones: Group of buildings within a region, provides isolation from building-level problems
  • Regions: Separate physical locations with considerable distances, useful for disaster recovery

Proximity and Separation Effects

• Proximity as a side effect of separation techniques:
  • Availability Sets give some proximity within clusters
  • Regions offer a ~2ms round trip latency envelope
• No SLA guaranteeing proximity

Proximity Placement Groups (PPGs)

• Purpose: Minimize latency between VMs and VM scale sets
• Applicability: Only for VMs and VM scale sets, not other services
• Expectation: Sub-millisecond latency (not guaranteed)

Working Mechanism of PPGs

• Creation and Pinning:
  1. Create a PPG in a chosen region (e.g., South Central US PPG1)
  2. PPG is initially floating in Azure Resource Manager, not pinned to a data center
  3. First resource added to PPG pins it to a specific data center
  4. All subsequent resources in that PPG will be in the same data center
• Availability and Capacity Constraint:
  • Capacity limited to the pinned data center
  • Supported VM SKUs depend on the specific data center
• Not About Disks: PPGs do not reduce latency between VMs and their disks

Best Practices for Using PPGs

• Declarative Technologies:
  • Use templates (ARM JSON, Bicep, Terraform) for VMs and PPGs
  • Helps Azure Resource Manager make informed placement decisions
• Non-Declarative Approaches:
  • Avoid using Powershell, CLI, or the portal for initial PPG setup
  • If necessary, start with the most exotic VM SKU to improve placement success

Handling Constraints and Errors

• Over-Constrained Allocation Request:
  • Indicates unmatched SKU combination in a single data center
  • Retry by reordering resources or use a declarative template

Additional Considerations

• Existing VMs:
  • Can de-allocate, add to PPG, and start up
  • Maintains alignment in the PPG
• Availability Sets in AZs via PPGs:
  • Allows placing availability sets in a specific Availability Zone
• Maintenance and Re-alignment:
  • Resources may show as non-aligned after maintenance
  • De-allocate and restart to re-align

Conclusion

• Purpose: PPGs aim to reduce VM-to-VM latency
• Not Influence on Disks: Does not influence VM-to-disk latency
• Best Practice: Use accelerated networking to further reduce latencies