Azure Dedicated Host is a service that provides physical servers - able to host one or more virtual machines - dedicated to one Azure subscription. Dedicated hosts are the same physical servers used in our data centers, provided as a resource. You can provision dedicated hosts within a region, availability zone, and fault domain. Then, you can place VMs directly into your provisioned hosts, in whatever configuration best meets your needs.

Benefits

Reserving the entire host provides the following benefits:

Hardware isolation at the physical server level. No other VMs will be placed on your hosts. Dedicated hosts are deployed in the same data centers and share the same network and underlying storage infrastructure as other, non-isolated hosts.

Control over maintenance events initiated by the Azure platform. While the majority of maintenance events have little to no impact on your Windows 365 Virtual Desktop, there are some sensitive workloads where each second of pause can have an impact. With dedicated hosts, you can opt in to a maintenance window to reduce the impact to your service.

With the Azure hybrid benefit, you can bring your own licenses for Windows and SQL to Azure. Using the hybrid benefits provides you with additional benefits. For more information, see Azure Hybrid Benefit.

Groups, hosts, and VMs

A host group is a resource that represents a collection of dedicated hosts. You create a host group in a region and an availability zone, and add hosts to it.

A host is a resource, mapped to a physical server in an Azure data center. The physical server is allocated when the host is created. A host is created within a host group. A host has a SKU describing which VM sizes can be created. Each host can host multiple VMs, of different sizes, as long as they are from the same size series.

High Availability considerations
For high availability, you should deploy multiple VMs, spread across multiple hosts (minimum of 2). With Azure Dedicated Hosts, you have several options to provision your infrastructure to shape your fault isolation boundaries.

Use Availability Zones for fault isolation
Availability zones are unique physical locations within an Azure region. Each zone is made up of one or more datacenters equipped with independent power, cooling, and networking. A host group is created in a single availability zone. Once created, all hosts will be placed within that zone. To achieve high availability across zones, you need to create multiple host groups (one per zone) and spread your hosts accordingly.

If you assign a host group to an availability zone, all VMs created on that host must be created in the same zone.

Use Fault Domains for fault isolation
A host can be created in a specific fault domain. Just like VM in a scale set or availability set, hosts in different fault domains will be placed on different physical racks in the data center. When you create a host group, you are required to specify the fault domain count. When creating hosts within the host group, you assign fault domain for each host. The VMs do not require any fault domain assignment.

Fault domains are not the same as colocation. Having the same fault domain for two hosts does not mean they are in proximity with each other.

Fault domains are scoped to the host group. You should not make any assumption on anti-affinity between two host groups (unless they are in different availability zones).

VMs deployed to hosts with different fault domains, will have their underlying managed disks services on multiple storage stamps, to increase the fault isolation protection.

Using Availability Zones and Fault Domains
You can use both capabilities together to achieve even more fault isolation. In this case, you will specify the availability zone and fault domain count in for each host group, assign a fault domain to each of your hosts in the group, and assign an availability zone to each of your VMs

The Resource Manager sample template uses zones and fault domains to spread hosts for maximum resiliency in a region.

Manual vs. automatic placement
When creating a VM in Azure, you can select which dedicated host to use. You can also use the option to automatically place your VMs on existing hosts, within a host group.

When creating a new host group, make sure the setting for automatic VM placement is selected. When creating your VM, select the host group and let Azure pick the best host for your VM.

Host groups that are enabled for automatic placement do not require all the VMs to be automatically placed. You will still be able to explicitly pick a host, even when automatic placement is selected for the host group.

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