Availability sets are an Azure feature that helps increase the resilience and uptime of virtual machines (VMs). They are especially important when running multiple VMs to support high-availability applications.
What Is an Availability Set?
An availability set is a group of Virtual Machines that Azure distributes across update domains and fault domains. This setup reduces the risk that all your VMs are affected by a single point of failure.
- Update Domains: These group VMs that can be rebooted at the same time during planned maintenance, like OS updates.
- Azure can assign up to 20 update domains.
- Azure waits at least 30 minutes before starting updates on another domain.
- Fault Domains: These group VMs that share a common power source and network switch.
- You can have up to 3 fault domains.
- A hardware issue in one domain won’t affect VMs in the others.
This design ensures your applications remain available during both planned and unplanned events.
Why Use Availability Sets?
If all your VMs are in a single fault or update domain, one issue could bring them all down. Availability sets reduce this risk by:
- Separating VMs across physical hardware.
- Ensuring not all VMs reboot at the same time.
This setup is vital for high-availability workloads like web servers or application tiers that require redundancy.
See also availability zones.
Service Level Agreement (SLA) Improvements
Using availability sets improves your SLA with Microsoft:
- Without availability sets, VM uptime is 95% to 99.9%.
- With availability sets, Azure provides an SLA of 99.95% uptime.
This increase in reliability is essential for business-critical services.
When to Use Availability Sets
- When you have 2 or more VMs running the same workload.
- When your app needs to stay online during updates or hardware failures.
- When your service must meet strict uptime guarantees.
For comparison with more flexible scaling solutions, see AZ-900: Virtual Machine Scale Sets.
Conclusion
Availability sets improve fault tolerance and uptime by spreading VMs across different physical and logical boundaries. They are a foundational feature for any high-availability Azure deployment.
Want to design resilient solutions from the ground up? Learn how in our AZ-900 video course. Alternatively, please go back to our list of AZ-900 requirements.
Please click here to find out more about Microsoft’s AZ-900 exam.
