We're excited to announce the public preview of automatic zone balance for Azure Virtual Machine Scale Sets. This new capability helps you maintain zone-resilient workloads with zero manual intervention. Automatic zone balance continuously monitors your scale set and redistributes VMs across availability zones, reducing the risk that a single zone failure disproportionately impacts your applications.
Maintain Optimal Resiliency Posture with Continuous Monitoring
When you deploy a Virtual Machine Scale Set across multiple availability zones, Azure spreads your VMs as evenly as possible for maximum resiliency. However, capacity constraints, scaling operations, and other factors can cause your scale set to become imbalanced over time, with some zones holding more VM instances than others. This imbalance often goes unnoticed, but it means a zone failure could take down a larger share of your workload than expected.
Figure 1: If an outage occurs in Zone 1, an imbalanced scale set would experience a 50% impact on its workload, while a balanced scale set would only experience 33% impact to an outage in any zone.Automatic zone balance addresses these challenges by continuously monitoring your scale set for zonal imbalances and tracking capacity in under-provisioned zones. When capacity becomes available, VMs are automatically created in the under-provisioned zone, eliminating the need for manual trial-and-error.
When rebalancing occurs, Automatic zone balance uses a create-before-delete approach. New VMs are created in under-provisioned zones and verified healthy before VMs in over-provisioned zones are removed, ensuring your workload capacity is never reduced during the process. Health checks are performed through integration with Application Health Extension or Load Balancer Health Probes, so only healthy VMs are kept.
Built-in safety guardrails ensure rebalancing respects instance protection policies and pauses during active scale set operations. To minimize churn, rebalancing includes back-off periods between operations and moves only one VM at a time.
When you enable Automatic zone balance, automatic instance repairs is also activated by default, giving you both zone-level resiliency and instance-level health monitoring. Together, these capabilities help you maintain resilient, well-distributed workloads with minimal operational overhead, reducing the blast radius of zone failures while ensuring gradual, controlled changes to your scale set.
How to Get Started
To get started with Automatic zone balance:
- Register for the preview: Enable the AutomaticZoneRebalancing feature flag in your subscription via Azure portal, CLI, or PowerShell.
- Ensure prerequisites: Your scale set must span at least 2 availability zones, use best-effort zone balancing mode, and have application health monitoring configured.
- Enable Automatic zone balance: Turn on the feature through Azure portal, CLI, PowerShell, or REST API.
For detailed instructions, visit the automatic zone balance documentation.
Microsoft