How to use availability sets

In this tutorial, you learn how to increase the availability and reliability of your Virtual Machine solutions on Azure using a capability called Availability Sets. Availability sets ensure that the VMs you deploy on Azure are distributed across multiple isolated hardware nodes in a cluster. Doing this ensures that if a hardware or software failure within Azure happens, only a subset of your VMs are impacted and that your overall solution remains available and operational.

In this tutorial, you learn how to:

  • Create an availability set
  • Create a VM in an availability set
  • Check available VM sizes
  • Check Azure Advisor

Launch Azure Cloud Shell

The Azure Cloud Shell is a free interactive shell that you can use to run the steps in this article. It has common Azure tools preinstalled and configured to use with your account. Just click the Copy to copy the code, paste it into the Cloud Shell, and then press enter to run it. There are a few ways to launch the Cloud Shell:

Click Try It in the upper right corner of a code block. Cloud Shell in this article
Open Cloud Shell in your browser. https://shell.azure.com/powershell
Click the Cloud Shell button on the menu in the upper right of the Azure portal. Cloud Shell in the portal

If you choose to install and use the PowerShell locally, this tutorial requires the Azure PowerShell module version 5.3 or later. Run Get-Module -ListAvailable AzureRM to find the version. If you need to upgrade, see Install Azure PowerShell module. If you are running PowerShell locally, you also need to run Login-AzureRmAccount to create a connection with Azure.

Availability set overview

An Availability Set is a logical grouping capability that you can use in Azure to ensure that the VM resources you place within it are isolated from each other when they are deployed within an Azure datacenter. Azure ensures that the VMs you place within an Availability Set run across multiple physical servers, compute racks, storage units, and network switches. If a hardware or Azure software failure occurs, only a subset of your VMs are impacted, and your overall application stays up and continues to be available to your customers. Availability Sets are an essential capability when you want to build reliable cloud solutions.

Let’s consider a typical VM-based solution where you might have four front-end web servers and use 2 back-end VMs that host a database. With Azure, you’d want to define two availability sets before you deploy your VMs: one availability set for the web tier and one availability set for the database tier. When you create a new VM you can then specify the availability set as a parameter to the az vm create command, and Azure automatically ensures that the VMs you create within the available set are isolated across multiple physical hardware resources. If the physical hardware that one of your Web Server or Database Server VMs is running on has a problem, you know that the other instances of your Web Server and Database VMs remain running because they are on different hardware.

Use Availability Sets when you want to deploy reliable VM-based solutions in Azure.

Create an availability set

You can create an availability set using New-AzureRmAvailabilitySet. In this example, set both the number of update and fault domains at 2 for the availability set named myAvailabilitySet in the myResourceGroupAvailability resource group.

Create a resource group.

New-AzureRmResourceGroup -Name myResourceGroupAvailability -Location EastUS

Create a managed availability set using New-AzureRmAvailabilitySet with the -sku aligned parameter.

New-AzureRmAvailabilitySet `
   -Location "EastUS" `
   -Name "myAvailabilitySet" `
   -ResourceGroupName "myResourceGroupAvailability" `
   -Sku aligned `
   -PlatformFaultDomainCount 2 `
   -PlatformUpdateDomainCount 2

Create VMs inside an availability set

VMs must be created within the availability set to make sure they are correctly distributed across the hardware. You can't add an existing VM to an availability set after it is created.

The hardware in a location is divided in to multiple update domains and fault domains. An update domain is a group of VMs and underlying physical hardware that can be rebooted at the same time. VMs in the same fault domain share common storage as well as a common power source and network switch.

When you create a VM with New-AzureRmVM, you use the -AvailabilitySetName parameter to specify the name of the availability set.

First, set an administrator username and password for the VM with Get-Credential:

$cred = Get-Credential

Now create two VMs with New-AzureRmVM in the availability set.

for ($i=1; $i -le 2; $i++)
{
    New-AzureRmVm `
        -ResourceGroupName "myResourceGroupAvailability" `
        -Name "myVM$i" `
        -Location "East US" `
        -VirtualNetworkName "myVnet" `
        -SubnetName "mySubnet" `
        -SecurityGroupName "myNetworkSecurityGroup" `
        -PublicIpAddressName "myPublicIpAddress$i" `
        -AvailabilitySetName "myAvailabilitySet" `
        -Credential $cred
}

The -AsJob parameter creates the VM as a background task, so the PowerShell prompts return to you. You can view details of background jobs with the Job cmdlet. It takes a few minutes to create and configure both VMs. When finished, you have two virtual machines distributed across the underlying hardware.

If you look at the availability set in the portal by going to Resource Groups > myResourceGroupAvailability > myAvailabilitySet, you should see how the VMs are distributed across the two fault and update domains.

Availability set in the portal

Check for available VM sizes

You can add more VMs to the availability set later, but you need to know what VM sizes are available on the hardware. Use Get-AzureRMVMSize to list all the available sizes on the hardware cluster for the availability set.

Get-AzureRmVMSize `
   -ResourceGroupName "myResourceGroupAvailability" `
   -AvailabilitySetName "myAvailabilitySet"

Check Azure Advisor

You can also use Azure Advisor to get more information on ways to improve the availability of your VMs. Azure Advisor helps you follow best practices to optimize your Azure deployments. It analyzes your resource configuration and usage telemetry and then recommends solutions that can help you improve the cost effectiveness, performance, high availability, and security of your Azure resources.

Sign in to the Azure portal, select All services, and type Advisor. The Advisor dashboard displays personalized recommendations for the selected subscription. For more information, see Get started with Azure Advisor.

Next steps

In this tutorial, you learned how to:

  • Create an availability set
  • Create a VM in an availability set
  • Check available VM sizes
  • Check Azure Advisor

Advance to the next tutorial to learn about virtual machine scale sets.