Enabling long file paths on Azure Service Fabric VMSS cluster - azure

Question

My Azure Service Fabric application sometimes requires paths longer than MAX_PATH, especially given the length of the work directory. As such, I'd like to enable long file paths (via the registry's LongPathsEnabled value, via group policy, or via some other mechanism, see https://superuser.com/questions/1119883/windows-10-enable-ntfs-long-paths-policy-option-missing). But I can't figure out how to do that.
The cluster runs on an Azure VMSS, so I can remote into the individual instances and set it manually, but that doesn't scale well of course.
UPDATE:
#4c74356b41's answer got me most of where I needed to be. My VMSS already had a customScript extension installed, so I actually had to modify it to include the PS command, here's my final command:
# Get the existing VMSS configuration
$vmss = Get-AzVmss -ResourceGroupName <resourceGroup> -Name <vmss>
# inspect $vmss to determine which extension is the customScript, in ours it's at index 3. Note the existing commandToExecute blob, you'll need to modify it to add the additional PS command
# modify the existing Settings.commandToExecute blob to add the reg set command
$vmss.VirtualMachineProfile.ExtensionProfile.Extensions[3].Settings.commandToExecute = 'powershell -ExecutionPolicy Unrestricted -File AzureQualysCloudAgentPowerShell_v2.ps1 && powershell -c "Set-ItemProperty -Path HKLM:\System\ControlSet001\Control\FileSystem -Name LongPathsEnabled -Value 1"'
# update the VMSS with the new config
Update-AzVmss -ResourceGroupName $vmss.ResourceGroupName -Name $vmss.Name -VirtualMachineScaleSet $vmss

Answer 1

I'd suggest using script extension and a simple powershell script to set this value. this will automatically get applied to all the instances (including to when you scale).
{
"apiVersion": "2018-06-01",
"type": "Microsoft.Compute/virtualMachineScaleSet/extensions",
"name": "config-app",
"location": "[resourceGroup().location]",
"properties": {
"publisher": "Microsoft.Compute",
"type": "CustomScriptExtension",
"typeHandlerVersion": "1.9",
"autoUpgradeMinorVersion": true,
"settings": {
"fileUris": []
},
"protectedSettings": {
"commandToExecute": "powershell -c 'Set-Item HKLM:\System\CurrentControlSet\Policies\LongPathsEnabled -Value 1'"
}
}
}
The command itself is probably a bit off, but you can experiment on your local and get it right and then put it into the script extension
https://learn.microsoft.com/en-us/azure/virtual-machines/extensions/custom-script-windows