Availability Sets and Availability Zones

When running production workloads on Azure VMs, you need to protect against failures. Azure provides two primary mechanisms for ensuring virtual machine availability: Availability Sets and Availability Zones. Understanding the difference between them — and when to use each — is essential for designing resilient architectures.

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Types of Failures

Before discussing availability features, it's important to understand what can go wrong:

Failure Type	Description	Example
Hardware failure	A physical component fails	Server power supply fails, memory error
Rack failure	An entire rack loses power or connectivity	Top-of-rack switch failure, rack PDU failure
Data centre failure	An entire facility is impacted	Power outage, cooling failure, natural disaster
Planned maintenance	Azure updates host OS or firmware	Monthly security patches, hardware upgrades

Azure's availability features are designed to mitigate these scenarios.

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Availability Sets

An Availability Set distributes VMs across multiple fault domains and update domains within a single data centre.

Fault Domains (FDs)

A fault domain represents a group of VMs that share a common power source and network switch — essentially a physical rack. If a rack fails, only VMs in that fault domain are affected.

• An Availability Set can have up to 3 fault domains (varies by region)
• Azure automatically distributes your VMs across fault domains

Update Domains (UDs)

An update domain represents a group of VMs that can be rebooted at the same time during planned maintenance. Azure updates one update domain at a time, ensuring that not all your VMs are rebooted simultaneously.

• An Availability Set can have up to 20 update domains (default is 5)
• During planned maintenance, only one UD is taken offline at a time

How It Works

Availability Set (3 FDs, 5 UDs)
┌─────────────────────────────────────────────────┐
│  FD 0         FD 1         FD 2                  │
│  ┌─────┐    ┌─────┐    ┌─────┐                  │
│  │ VM1 │    │ VM2 │    │ VM3 │    ← UD 0,1,2   │
│  │ VM4 │    │ VM5 │    │ VM6 │    ← UD 3,4,0   │
│  └─────┘    └─────┘    └─────┘                  │
│  (Rack A)   (Rack B)   (Rack C)                 │
└─────────────────────────────────────────────────┘

Creating an Availability Set

az vm availability-set create \
  --resource-group rg-ha-demo \
  --name myAvailabilitySet \
  --platform-fault-domain-count 3 \
  --platform-update-domain-count 5

SLA

Deploying 2 or more VMs in an Availability Set provides a 99.95% SLA for compute.

Limitations

• VMs must be in the same region and resource group
• VMs must be added to the Availability Set at creation time (cannot be added later)
• Protects against rack-level failures only — not against data centre-level failures
• All VMs are still within the same data centre

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Availability Zones

Availability Zones are physically separate locations within an Azure region. Each zone has independent power, cooling, and networking. They provide protection against data centre-level failures.

How Availability Zones Work

Most Azure regions that support Availability Zones have three zones: