Boot diagnostics disabled: when the VM won't start, you're debugging a black box

Why it happens

Boot diagnostics is off unless explicitly enabled, and many ARM templates, Terraform modules, and image pipelines never set it — it's a nice-to-have that adds nothing on the happy path, so it gets skipped. The portal enables it for portal-created VMs, which makes coverage inconsistent: hand-made VMs have it, the infrastructure-as-code fleet often doesn't.

The trap has a nasty asymmetry: the setting only matters when the VM is already unbootable, and that's precisely when you can no longer turn it on and see what happened. The evidence you needed was never captured. Everything before that moment looks fine, so nothing prompts anyone to check.

What it costs / blast radius

With the managed storage option, boot diagnostics costs effectively nothing — no storage account to maintain, no meaningful line on the bill. This is pure operational insurance. What's at stake is incident time: a VM stuck at boot with diagnostics enabled shows you the kernel panic, the BSOD stop code, or the fsck prompt in one screenshot; the same VM without it is a black box you resort to detaching disks and rebuilding around, while the outage runs. The check is hygiene, but the moment it matters, it really matters. (Authored assessment of Azure behavior.)

See it

resource vm 'Microsoft.Compute/virtualMachines@2024-07-01' = {
  name: 'app-vm-01'
  location: location
  properties: {
    hardwareProfile: { vmSize: 'Standard_D4s_v5' }
    // ...osProfile and networkProfile omitted for brevity
    // diagnosticsProfile omitted: no console screenshot,
    // no serial log when this VM fails to start
  }
}

resource vm 'Microsoft.Compute/virtualMachines@2024-07-01' = {
  name: 'app-vm-01'
  location: location
  properties: {
    hardwareProfile: { vmSize: 'Standard_D4s_v5' }
    // ...osProfile and networkProfile omitted for brevity
    diagnosticsProfile: {
      bootDiagnostics: {
        enabled: true   // omit storageUri to use managed storage
      }
    }
  }
}