Learn how to work with Availability Sets on Azure

What Are Availability Sets in Azure?

Availability Sets in Azure are a feature designed to increase the availability and resiliency of your Virtual Machines (VMs) by ensuring that they are distributed across multiple fault domains and update domains within a data center.

This configuration protects your applications from both hardware failures and planned maintenance.

When you deploy VMs in an Availability Set, Azure guarantees that your VMs will be distributed in such a way that a failure in one part of the infrastructure won’t bring down all of your VMs at once.

Key Concepts Related to Availability Sets

Fault Domain

A Fault Domain (FD) is a group of physical hardware in an Azure data center, such as a rack of servers or a group of power and network resources.

Fault domains help to protect against hardware failures.

Example:

If a power supply fails in one fault domain, Azure will automatically place your VMs in different fault domains, ensuring that not all VMs are affected.

When you use Availability Sets, Azure automatically distributes your VMs across multiple fault domains.

Update Domain

An Update Domain (UD) is a logical group of VMs that are rebooted during a planned update or maintenance event.

During a platform maintenance event (such as patching or upgrading the underlying Azure infrastructure), VMs in different update domains will be rebooted at different times.

This ensures that at least one instance of your application is always available during updates, as not all VMs are taken down at once.

Example:

If Azure is performing maintenance on a subset of VMs, only VMs in a specific update domain will be impacted.

Minimum Number of VMs in an Availability Set

To achieve high availability, it’s recommended to have at least two VMs in an Availability Set.

However, for better redundancy, you can add more VMs (e.g., 3 or 5) to better distribute your VMs across fault and update domains.

For optimal performance, try to align the number of VMs with the desired level of redundancy.

Availability Set vs. Availability Zones

Availability Sets provide redundancy within a single data center (region), while Availability Zones distribute VMs across physically separate data centers in a region.

Availability Zones provide higher resilience because they are isolated not just by fault and update domains, but by physical location (e.g., separate power sources, network connectivity, and cooling).

Key Difference:

Availability Zones are more resilient against regional outages and large-scale failures compared to Availability Sets, which are confined to a single data center.

Key Benefits of Using Availability Sets

High Availability

By distributing VMs across multiple fault domains and update domains, you reduce the risk of having all your VMs fail simultaneously due to hardware failures or scheduled platform maintenance.

Even during an Azure platform update, only a subset of your VMs will be impacted, ensuring that your application remains available.

Protection Against Hardware Failures

If there is a failure in a fault domain (e.g., a hardware failure, power outage, or network failure in one rack), VMs in other fault domains will not be affected, ensuring service continuity.

Planned Maintenance and Patching

Availability Sets enable rolling updates, where VMs are updated one update domain at a time.

This minimizes downtime during Azure platform maintenance or when applying patches to your VMs.

Cost-Effective

Availability Sets are included with your Azure subscription at no additional cost, making them a cost-effective way to achieve higher availability without the need for complex infrastructure setups.

Improved SLA

Azure provides a higher SLA (Service Level Agreement) for VMs deployed in an Availability Set.

The SLA for uptime of your VMs increases to 99.95% for two or more VMs deployed in an Availability Set.

Without an Availability Set, the SLA is only 99.9%.

How Availability Sets Work

When you create a Virtual Machine in Azure, you can specify that it should be deployed in an Availability Set.

Here's how the system ensures availability.

VM Distribution

Once your VMs are part of an Availability Set, Azure automatically distributes the VMs across fault domains and update domains.

The VMs will be distributed so that at least one VM will be available during hardware failures (due to fault domain isolation) and during planned updates (due to update domain isolation).

Fault Domains

Each VM is placed in a different fault domain, meaning they are hosted on different physical servers with independent power supplies, network switches, etc.

If one fault domain experiences a failure (e.g., a hardware failure), only the VMs in that fault domain will be impacted.

Update Domains

VMs are assigned to different update domains, ensuring that only one domain is updated at a time during planned maintenance events.

If Azure needs to reboot or patch one update domain, the other domains remain unaffected, so your app stays available.

Best Practices for Using Availability Sets

Use at Least Two VMs

For high availability, deploy at least two VMs in an Availability Set.

This ensures that your application remains available even if one VM fails.

Deploy more VMs (e.g., three or five) for better fault tolerance and to take advantage of the full benefits of Availability Sets.

Balance Across Fault and Update Domains

Ensure that your VMs are distributed across multiple fault domains and update domains to avoid single points of failure and to ensure continuous availability during maintenance.

Combine with Other Resiliency Features

Use Azure Load Balancer or Application Gateway to distribute traffic between VMs, so if one VM goes down, the other VMs can take over.

If your application is mission-critical, consider combining Availability Sets with Azure Site Recovery (ASR) for disaster recovery and replicating VMs to other regions.

Automate Patching

Leverage Azure Automation Update Management to automate and schedule patching, which helps ensure that your VMs remain up-to-date with security updates, without taking the entire application offline.

Avoid Single Points of Failure

While Availability Sets ensure high availability within a single region, consider Availability Zones if your application demands greater resilience across multiple data centers.

Limitations of Availability Sets

While Availability Sets are a powerful feature, there are some limitations to consider:

Single Region Limitation

Availability Sets provide redundancy within a single Azure region.

They don’t protect against regional outages or large-scale infrastructure failures that affect the entire region.

Limited to Virtual Machines

Availability Sets are designed for VMs but not for other Azure resources like Azure App Services, Azure Functions, or Azure Kubernetes Service (AKS).

If you want more extensive redundancy, you might need to use Availability Zones for those services.

VM Type Restrictions

Classic VMs (based on the older Azure Service Management model) cannot be placed in Availability Sets, as Availability Sets are a feature of the Azure Resource Manager (ARM) model.

Not Available for All VM Sizes

Availability Sets have some restrictions on the types of VM sizes that can be placed within them.

Certain specialized VM types (e.g., Azure GPU VMs) may not support Availability Sets in some regions.

Creating and Managing Availability Sets

How to Create an Availability Set

Azure Portal

1. Navigate to the Azure Portal > Virtual Machines.

2. When creating a VM, in the Availability Options section, select Availability Set and either choose an existing set or create a new one.

Azure CLI or PowerShell

You can also create Availability Sets using the Azure CLI or PowerShell with the following commands:

CLI Example:

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az vm availability-set create \
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--name myAvailabilitySet \
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--resource-group myResourceGroup \
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--platform-fault-domain-count 2 \
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--platform-update-domain-count 2

PowerShell Example:

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New-AzAvailabilitySet `
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-ResourceGroupName myResourceGroup `
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-Name myAvailabilitySet `
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-FaultDomainCount 2 `
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-UpdateDomainCount 2

Automatic Distribution:

When VMs are deployed to an Availability Set, Azure automatically distributes them across fault and update domains based on the number of VMs and the available domains.

Summary

Availability Sets are a fundamental feature of Azure that ensures the availability and resiliency of your VMs in the face of hardware failures and planned maintenance.

By distributing your VMs across multiple fault and update domains, you reduce the risk of downtime and ensure continuous service even during Azure platform updates or infrastructure issues.

However, if your application requires even higher levels of resiliency (e.g., protection from regional outages), consider using Availability Zones or multi-region deployment.

For most applications that don't need zone-level redundancy, Availability Sets provide an effective and cost-efficient solution to maintain high availability.