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What are Availability Sets?

Availability Sets are a logical grouping of two or more Azure VM’s. It is advised to use an Availability Set for each application tier. For example,  placing Domain Controllers in one Availability Set, SQL Servers in a second, and IIS web servers in a third. If we did not use Availability Sets, Microsoft Azure has no way of knowing the different application tiers for each Virtual Machine, because of this you application/infrastructure can fail. This failure can be from planned maintenance from Microsoft rebooting all your VM’s or failure in the hardware infrastructure causing an outage.

What are Update Domain’s and Fault Domain’s?

Fault domain: Fault Domains are physical points of failure. In Azure, this is basically anything that could affect the Hyper-V hosts such as Network or Power failure. All Virtual Machines in a fault domain share the underlying infrastructure. With each availability set, we have two fault domains by default.

Update Domain: Update Domains are logical points of failure. Update domains exist to allow Microsoft to update and reboot the physical Hyper-V hosts without affecting your application/infrastructure. Each availability set has five update domains and VMs are grouped into these automatically in a sequential manner. This is important to note when designing your environment.

Create Azure Availability Set

There are two ways you can create an Availability Set using the Portal. The first way is when you are creating a new VM. The second is to create the availability set first and then assign a VM to it. This guide will show you how to create a new empty availability set.

Log in to your Azure Subscription by going to https://portal.azure.com

Click the green + and then Search for Availability Set

Click Availability Set

Click Create

Enter a Name, Select the Subscription you would like to use, either create a new Resource group or use an existing one and select the Location.

You can then select how many Fault Domains and Update Domains you need.

You should have something like this:

Make sure everything is ok and then click Create

You have now created an empty Availability set ready for your VM’s.

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To move a VM in or out of an availability set, you need to recreate it.
You can place a maximum of 100 VMs into each availability set.
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