Availability options for virtual machines in Azure

This article provides you with an overview of the availability features of Azure virtual machines (VMs).

High availability

Workloads are typically spread across different virtual machines to gain high throughput, performance, and to create redundancy in case a VM is impacted due to an update or other event.

There are few options that Azure provides to achieve High Availability. First let’s talk about basic constructs.

Availability zones

Availability zones expand the level of control you have to maintain the availability of the applications and data on your VMs. An Availability Zone is a physically separate zone, within an Azure region. There are three Availability Zones per supported Azure region.

Each Availability Zone has a distinct power source, network, and cooling. By architecting your solutions to use replicated VMs in zones, you can protect your apps and data from the loss of a datacenter. If one zone is compromised, then replicated apps and data are instantly available in another zone.

Availability zones

Learn more about deploying a Windows or Linux VM in an Availability Zone.

Fault domains

A fault domain is a logical group of underlying hardware that share a common power source and network switch, similar to a rack within an on-premises datacenter.

Update domains

An update domain is a logical group of underlying hardware that can undergo maintenance or be rebooted at the same time.

This approach ensures that at least one instance of your application always remains running as the Azure platform undergoes periodic maintenance. The order of update domains being rebooted may not proceed sequentially during maintenance, but only one update domain is rebooted at a time.

Virtual Machines Scale Sets

Azure virtual machine scale sets let you create and manage a group of load balanced VMs. The number of VM instances can automatically increase or decrease in response to demand or a defined schedule. Scale sets provide high availability to your applications, and allow you to centrally manage, configure, and update many VMs. We recommended that two or more VMs are created within a scale set to provide for a highly available application and to meet the 99.95% Azure SLA. There is no cost for the scale set itself, you only pay for each VM instance that you create. When a single VM is using Azure premium SSDs, the Azure SLA applies for unplanned maintenance events. Virtual machines in a scale set can be deployed across multiple regions and fault domains to maximize availability and resilience to outages due to data center outages, and planned or unplanned maintenance events. Virtual machines in a scale set can also be deployed into a single Availability zone, or regionally. Availability zone deployment options may differ based on the orchestration mode.

Preview: Orchestration mode Preview

Virtual machines scale sets allow you to specify orchestration mode. With the virtual machine scale set orchestration mode (preview), you can now choose whether the scale set should orchestrate virtual machines which are created explicitly outside of a scale set configuration model, or virtual machine instances created implicitly based on the configuration model. Choose the orchestration mode that VM orchestration model allows you group explicitly defined Virtual Machines together in a region or in an availability zone. Virtual machines deployed in an Availability Zone provides zonal isolation to VMs are they are bound to the availability zone boundary and are not subjected to any failures that may occur in other availability zone in the region.

“orchestrationMode”: “VM” (VirtualMachine) “orchestrationMode”: “ScaleSetVM” (VirtualMachineScaleSetVM)
VM configuration model None. VirtualMachineProfile is undefined in the scale set model. Required. VirtualMachineProfile is populated in the scale set model.
Adding new VM to Scale Set VMs are explicitly added to the scale set when the VM is created. VMs are implicitly created and added to the scale set based on the VM configuration model, instance count, and AutoScaling rules.
Availability Zones Supports regional deployment or VMs in one Availability Zone Supports regional deployment or multiple Availability Zones; Can define the zone balancing strategy
Fault domains Can define fault domains count. 2 or 3 based on regional support and 5 for Availability zone. The assigned VM fault domain will persist with VM lifecycle, including deallocate and restart. Can define 1, 2, or 3 fault domains for non-zonal deployments, and 5 for Availability zone deployments. The assigned VM fault domain does not persist with VM lifecycle, virtual machines are assigned a fault domain at time of allocation.
Update domains N/A. Update domains are automatically mapped to fault domains N/A. Update domains are automatically mapped to fault domains

Fault domains and update domains

Virtual machine scale sets simplify designing for high availability by aligning fault domains and update domains. You will only have to define fault domains count for the scale set. The number of fault domains available to the scale sets may vary by region. See Number of Fault Domains per region.

Availability sets

An availability set is a logical grouping of VMs within a datacenter that allows Azure to understand how your application is built to provide for redundancy and availability. We recommended that two or more VMs are created within an availability set to provide for a highly available application and to meet the 99.95% Azure SLA. There is no cost for the Availability Set itself, you only pay for each VM instance that you create. When a single VM is using Azure premium SSDs, the Azure SLA applies for unplanned maintenance events.

In an availability set, VMs are automatically distributed across these fault domains. This approach limits the impact of potential physical hardware failures, network outages, or power interruptions.

For VMs using Azure Managed Disks, VMs are aligned with managed disk fault domains when using a managed availability set. This alignment ensures that all the managed disks attached to a VM are within the same managed disk fault domain.

Only VMs with managed disks can be created in a managed availability set. The number of managed disk fault domains varies by region - either two or three managed disk fault domains per region. You can read more about these managed disk fault domains for Linux VMs or Windows VMs.

Managed availability set

VMs within an availability set are also automatically distributed across update domains.

Availability sets

Next steps

You can now start to use these availability and redundancy features to build your Azure environment. For best practices information, see Azure availability best practices.