availability-zones

name	availability-zones
description	VM AZ design — zonal vs zone-redundant, latency, AZ-aware LB, SLA

Availability Zones

When to use this skill

• Greenfield design
• Targeting 99.99% VM SLA

Key concepts

• AZ regions provide ≥3 physically separated zones with independent power/cooling/network.
• Zonal VM is pinned to one zone. Zone-redundant isn't a VM concept — services like Standard LB and managed disks (ZRS) are.
• Spread VMs across ≥2 zones behind a zone-redundant Standard LB for 99.99% SLA.
• Cross-zone latency typically 1–2 ms — usually fine, but chatty east-west needs design awareness.
• Subscription-to-physical zone mapping differs per sub — 'zone 1' in your sub may differ from another's.

Azure CLI examples

az vm create -g <rg> -n vm-app-z1 --image Ubuntu2204 \
  --zone 1 --size Standard_D2s_v5 ...

Bicep example

resource vm 'Microsoft.Compute/virtualMachines@2024-03-01' = {
  name: 'vm-app-z1'
  location: location
  zones: [ '1' ]
  properties: { /* ... */ }
}

Common pitfalls

• Deploying all VMs to zone 1 — defeats the purpose.
• Mixing Basic LB with zonal VMs — Basic LB isn't zone-aware.
• Assuming logical zone numbers map identically across subscriptions.

References

• Microsoft Learn: search for "Availability Zones Azure VM" on https://learn.microsoft.com

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