[Feature Request] Support Aligned Zonal Fault Domains for VMSS Flex

[fitzgeraldsteele]

Service Team Request

API Spec: Azure/azure-rest-api-specs#39109
Feature Description: Aligned Zonal Fault Domains for VMSS Flex — enables 1:1 mapping between compute fault domains and storage fault partitions within a single Availability Zone
Target Service Public Preview: 2026-06-30
Target CLI Release: CLI 2.87.0 — 2026-06-02

Updated 2026-05-04 — design has firmed up; this issue body now reflects the final cmdlet/CLI surface as agreed with the service team. The corresponding PowerShell design review is Azure/azure-powershell-cmdlet-review-pr#1548.

---

Preview Requirements

⚠️ Feature Flag Required During Preview
Register the following AFEC on your subscription:

az feature register --namespace Microsoft.Compute --name ZonalAlignedMultipleFDs
az provider register --namespace Microsoft.Compute

---

Requested Commands

New Parameters for Existing Commands

Command	Parameter	Type	Allowed Values	Notes
az vmss create	--zonal-fault-domain-align-mode	string	Aligned, Unaligned, BestEffortAligned	VMSS-level alignment mode
az vmss create	--os-disk-storage-fd-alignment	string	Aligned, BestEffortAligned	Per-disk override. Omit to leave unaligned.
az vmss create	--data-disk-storage-fd-alignment	string	Aligned, BestEffortAligned	Per-disk override. Omit to leave unaligned.
az vm create	--os-disk-storage-fd-alignment	string	Aligned, BestEffortAligned	Only valid when --vmss joins a Flex VMSS.
az vm create	--data-disk-storage-fd-alignment	string	Aligned, BestEffortAligned	Only valid when --vmss joins a Flex VMSS.

Important Constraints

1. Per-disk values do NOT include Unaligned. CRP returns BadRequest if Unaligned is sent at the disk level. To leave a disk unaligned, omit the parameter — do not pass Unaligned.
2. Per-disk alignment is only valid for VMs in a Flex VMSS. CRP returns BadRequest if either per-disk parameter is set on a standalone VM not joined to a Flex VMSS.
3. Per-disk alignment is set-at-create only — values cannot be changed via update.
4. VMSS-level alignment IS updatable via az vmss update --set zonalPlatformFaultDomainAlignMode=....
5. Flexible orchestration only. Aligned/BestEffortAligned modes are not supported for Uniform VMSS.
6. Single zone only. Alignment only applies when the VMSS is deployed to a single Availability Zone.

No New Commands Required

Updates leverage generic --set for az vmss update.

---

Example Usage

# 1. Create VMSS with full (strict) alignment
az vmss create \
  --name myVmss \
  --resource-group myRg \
  --location eastus2 \
  --zones 1 \
  --orchestration-mode Flexible \
  --platform-fault-domain-count 3 \
  --zonal-fault-domain-align-mode Aligned \
  --image Ubuntu2204 \
  --vm-sku Standard_D4s_v5 \
  --instance-count 3

# 2. Create VMSS with best-effort alignment + per-disk override on OS disk
az vmss create \
  --name myVmss \
  --resource-group myRg \
  --location eastus2 \
  --zones 1 \
  --orchestration-mode Flexible \
  --platform-fault-domain-count 3 \
  --zonal-fault-domain-align-mode BestEffortAligned \
  --os-disk-storage-fd-alignment BestEffortAligned \
  --image Ubuntu2204 \
  --vm-sku Standard_D4s_v5

# 3. Constrained disk-type case: OS disk on Premium LRS forced aligned;
#    data disk on Premium SSD v2 left unaligned by OMITTING the parameter
az vmss create \
  --name myVmss \
  --resource-group myRg \
  --location eastus2 \
  --zones 1 \
  --orchestration-mode Flexible \
  --platform-fault-domain-count 3 \
  --zonal-fault-domain-align-mode BestEffortAligned \
  --os-disk-storage-fd-alignment Aligned \
  --image Ubuntu2204 \
  --vm-sku Standard_D4s_v5
# (Do NOT pass --data-disk-storage-fd-alignment Unaligned — that fails. Omit it.)

# 4. Update existing VMSS (only VMSS-level mode is updatable)
az vmss update \
  --name myVmss \
  --resource-group myRg \
  --set zonalPlatformFaultDomainAlignMode=BestEffortAligned

# 5. Query disk alignment status (read-only) via VM instance view
az vm get-instance-view \
  --ids $(az vmss list-instances --resource-group myRg --name myVmss --query "[].id" -o tsv) \
  --query "{vm:name, disks:instanceView.disks[].{name:name, alignmentState:storageAlignmentStatus}}"

---

API Properties

Model	Property	Type	Allowed Values	Notes
VirtualMachineScaleSet.properties	zonalPlatformFaultDomainAlignMode	Enum	Aligned, Unaligned, BestEffortAligned	Updatable
VirtualMachineScaleSetUpdate.properties	zonalPlatformFaultDomainAlignMode	Enum	same as above	Update path
VirtualMachineScaleSetOSDisk	storageFaultDomainAlignment	Enum	Aligned, BestEffortAligned	Per-disk; create-only; omit for unaligned
VirtualMachineScaleSetDataDisk	storageFaultDomainAlignment	Enum	Aligned, BestEffortAligned	Per-disk; create-only; omit for unaligned
OSDisk (VM)	storageFaultDomainAlignment	Enum	Aligned, BestEffortAligned	Flex VMSS member only
DataDisk (VM)	storageFaultDomainAlignment	Enum	Aligned, BestEffortAligned	Flex VMSS member only
DiskInstanceView (read-only)	storageAlignmentStatus	Enum	Aligned, Unaligned	Current alignment state

---

Dependencies

• Swagger PR: Add BestEffortAligned and PerDiskAlignment azure-rest-api-specs#39109
• Minimum API version: 2025-11-01 (was 2025-04-01 in the original request — updated)
• Related commands: az vmss create, az vmss update, az vm create, az vm get-instance-view, az disk show
• Orchestration mode: Flexible only
• Companion design review (PowerShell): Azure/azure-powershell-cmdlet-review-pr#1548

---

Contact

• PM: Jerry Steele (fisteele@microsoft.com / @fitzgeraldsteele)
• Engineering: Dominique Thomas (dothomas@microsoft.com / @Dthomas0729)
• Service Team: Azure Compute VM Team

[yonzhan]

Thank you for opening this issue, we will look into it.

[github-actions]

Here are some similar issues that might help you. Please check if they can solve your problem.

• Assign Fault Domain during VM Create #16346

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[yanzhudd]

May I confirm whether the Swagger is public? Azure CLI can only develop the feature if the Swagger is publicly available

[fitzgeraldsteele]

hi @yanzhudd. This will be available in the 2025-11-01 CRP API version which is currently working its way through approval, though not public yet. I put a link to the PR for this change above. The branch for the 2025-11-01 api version is here: https://github.com/Azure/azure-rest-api-specs/tree/cplat-2025-11-01