How Fault Domains Isolate Rack Hardware Failures

What Fault Domains Are

A fault domain is a logical representation of underlying physical hardware in an Azure datacenter that shares a common power source and network switch. When you place multiple virtual machines (VMs) in an availability set, Azure automatically distributes those VMs across different fault domains to avoid single points of failure.

How It Meets the Requirement

By assigning VMs to different fault domains, Azure ensures that an unexpected hardware failure affecting one physical rack does not bring down all instances of an application:

• Power isolation: If a power unit or rack power distribution fails, only VMs on that specific fault domain are impacted.
• Network isolation: If a top-of-rack switch encounters an outage, instances on other fault domains remain accessible.
• Hardware protection: Physical server failures remain isolated to the specific fault domain where the hardware resides.

Key Advantages and Capabilities

• Automatic distribution: Up to three fault domains can be used per availability set depending on the region.
• Storage alignment: Managed disks attached to VMs are automatically aligned with the same fault domain to prevent correlated storage failures.
• High availability SLA: Deploying two or more VMs across fault domains within an availability set qualifies the workload for a 99.95% uptime SLA.
• Zero additional cost: Availability sets and fault domains are platform features provided at no additional cost beyond normal VM compute charges.

Why This Is Optimal

Using fault domains provides essential physical hardware fault tolerance within a single datacenter, ensuring that unplanned hardware crashes or rack-level power outages do not disrupt the entire application.