Azure Availability Set

You need to have an availability set already defined before creating VMs in Azure.

So what does an availability set do?

• An availability set is a group of virtual machines that are deployed across fault domains and update domains.
• Availability sets make sure that your application is not affected by single points of failure, like the network switch or the power unit of a rack of servers.

Fault Domain:

Each VM in Azure is assigned to a fault domain – Fault domains define the group of virtual machines that share a common power source and network switch.

Update Domain:

• Virtual machines get update domains automatically once they are put inside availability set.
• All virtual machines within that update domain will reboot together.
• Update domains are used for patching of the virtual machines.
• Only one update domain would be updated at the time.
• In Azure Resource Manager(ARM) portal, we have up to three Fault domains and 5 update domains but we can have up to 20 update domains.
• How many Fault Domains and How many Update Domains is recommended? it depends !
  • If your region allows 3 fault domains, it is recommended to use 3, otherwise use 2 that is available to all regions.
  • 5 update domains is recommended. But if you choose to have more update domain, it will take longer to get all the update applied to your system.