Generally Available: Automatic Zone Balance for Virtual Machine Scale Sets

We’re excited to announce the general availability of automatic zone balance for Virtual Machine Scale Sets. Automatic zone balance continuously monitors your scale set and redistributes VM instances across availability zones, reducing the risk that a single-zone failure disproportionately impacts your applications.

Maintain optimal resiliency as your scale set evolves

When you deploy a Virtual Machine Scale Set across multiple availability zones, Azure attempts to spread your VMs as evenly as possible for maximum resiliency. However, capacity constraints, scaling operations, and other factors can cause your scale sets 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 and safety conditions are met, Azure automatically creates replacement VMs in under-provisioned zones and removes VMs from over-provisioned zones, eliminating the need for manual operations.

Rebalancing with built-in safeguards

Automatic zone balance uses a create-before-delete approach, creating replacement VMs in under-provisioned zones before removing VMs from over-provisioned zones. Application health checks performed through application health extension or load balancer health probes help protect application availability during rebalancing by evaluating the health of replacement VMs before existing instances are removed.

Built-in safety guardrails ensure rebalancing respects instance protection policies and doesn’t start during active or recently completed scale set operations. To minimize churn, rebalancing includes cooldown periods between operations and replaces only one VM at a time.

Automatic zone balance also enables automatic instance repairs by default, providing instance-level health monitoring and zone-level resiliency out of the box, with the option to opt out of repairs if you prefer to manage instance recovery independently.

Changes since preview

Since the public preview announcement, we’ve introduced the following improvements on automatic zone balance:

• Coordination with Azure Autoscale: If Autoscale triggers a scale-in during rebalancing, the scale set will complete the rebalance operation first before performing the scale-in. However, scale-out operations and customer-initiated capacity changes will take priority, cancelling any in-progress rebalance operations so the scale set can update to the requested capacity.
• Zone-balance calculations across VM power states: Active, deallocated, and powered-off VMs all count toward zone distribution. Rebalancing proceeds only when total instance counts are imbalanced and moving an active VM would improve active-instance distribution. See examples of how deallocated and powered-off VMs affect rebalancing.

How to Get Started

To get started with automatic zone balance:

1. Confirm workload suitability: Automatic zone balance is best suited for stateless workloads because rebalancing recreates VMs rather than preserving their identity, networking, and disks.
2. Check prerequisites: Configure at least two availability zones, ensure all instances are zonal, and enable application health monitoring.
3. Enable Automatic zone balance: Turn on the feature through the Azure portal, Azure CLI, PowerShell, or REST API. 

For detailed instructions and supported configurations, visit the Automatic zone balance overview and enablement guide.