We're excited to announce the public preview of availability set to Virtual Machine Scale Set migration for Azure Virtual Machines. This new capability enables you to move your existing VMs from availability sets to Virtual Machine Scale Sets with Flexible orchestration—unlocking higher availability, autoscaling, and zone-level resiliency without recreating your workloads from scratch.
For detailed instructions, see Migrate virtual machines from availability sets to Virtual Machine Scale Sets.
Why migrate from availability sets to Virtual Machine Scale Sets?
Virtual Machine Scale Sets with Flexible orchestration provide several advantages over availability sets:
| Capability | Virtual Machine Scale Sets (Flexible) | Availability Sets |
|---|---|---|
| Maximum instances | Up to 1,000 VMs | Up to 200 VMs |
| Availability zone support | Yes | No |
| Autoscaling | Yes | No |
| Rolling upgrades | Yes | No |
| Instance protection | Yes | No |
| Availability SLA | 99.95% (fault domains) or 99.99% (zones) | 99.95% |
How it works
The migration follows a structured, step-by-step process that gives you full control:
- Create a target Virtual Machine Scale Set with Flexible orchestration mode (regional or zonal)
- Validate that your availability set is eligible for migration
- Start migration to put the availability set into migration mode
- Migrate VMs individually to the target scale set
- Start VMs after each migration completes
- Clean up by deleting the empty availability set
VMs are migrated one at a time, giving you control over the pace and allowing you to verify each VM before proceeding. If something doesn't go as planned, you can cancel the migration at any point—VMs that haven't been migrated yet remain in the availability set.
Migrate directly from the Azure portal
You can also migrate entirely from the Azure portal with a guided experience that handles validation, scale set selection, zone assignment, and migration in a streamlined flow. Navigate to your availability set and select Migrate to VMSS Flex from the toolbar to get started. The Azure portal walks you through each step:
- Select or create a target scale set — Choose an existing compatible scale set from a dropdown, or quick-create a new one directly from the migration experience.
- Assign availability zones — If you selected a zonal scale set, assign each VM to a specific availability zone for maximum resiliency.
- Review and migrate — Review the configuration, then select Migrate to move all VMs at once.
- Start VMs and clean up — After migration completes, start your VMs and delete the empty availability set.
Note: The portal migrates all VMs in the availability set at the same time. If you need to migrate VMs one at a time to maintain application uptime during migration, use Azure CLI, PowerShell, or the REST API instead.
Regional and zonal migration paths
You can migrate to either a regional or zonal Virtual Machine Scale Set:
- Regional migration distributes VMs across fault domains within a region, similar to availability sets but with all the scale set benefits.
- Zonal migration places VMs into specific availability zones (1, 2, or 3), giving you the highest level of resiliency with a 99.99% SLA. You can also optionally change VM sizes during zonal migration.
We recommend zonal migration for workloads that need maximum resiliency. Distribute your VMs across multiple zones so that a single zone failure only impacts a fraction of your workload.
How to Get Started
To get started with availability set to Virtual Machine Scale Set migration:
- Register for the preview: Enable the MigrateToVmssFlex feature flag in your subscription via Azure CLI or PowerShell.
- Create a target scale set: Create a Virtual Machine Scale Set with Flexible orchestration mode in the same region as your availability set.
- Validate and migrate: Run the validation API against your availability set, then migrate your VMs one at a time to the target scale set.
For detailed instructions, see Migrate virtual machines from availability sets to Virtual Machine Scale Sets.
Microsoft