Azure Availability sets, Fault Domains, and Update Domains

Question

I'm Azure newbie and need some clarifications:

When adding machines to Availability set, in order to prevent VM from rebooting, what's best strategy for VM's, put them in:

-different update and fault domains

-same update domain

-same fault domain ?

My logic is that it's enough to put them in diffrent update AND fault domain

I used this as reference:https://blogs.msdn.microsoft.com/plankytronixx/2015/05/01/azure-exam-prep-fault-domains-and-update-domains/

Am i correct ?

These update/fault domains are confusing

Answer 1

My logic is that it's enough to put them in diffrent update AND fault domain

You are right, we should put VMs in different update and fault domain.

We put them in different update domain, when Azure hosts need update, Microsoft engineer will update one update domain, when it completed, update another update domain. In this way, our VMs will not reboot in the same time.

we put them in different fault domain, when an Unexpected Downtime happened, VMs in that fault domain will reboot, other VMs will keep running, in this way, our application running on those VMs will keep health.

To shot, add VMs to an availability set with different update domain and fault domain, that will get a high SLA, but not means one VM will not reboot.

Hope that helps.

Answer 2

There are three scenarios that can lead to virtual machine in Azure being impacted: unplanned hardware maintenance, unexpected downtime, and planned maintenance.

• Unplanned Hardware Maintenance
• An Unexpected Downtime
• Planned Maintenance events

Each virtual machine in your availability set is assigned an update domain and a fault domain by the underlying Azure platform. For a given availability set, five non-user-configurable update domains are assigned by default (Resource Manager deployments can then be increased to provide up to 20 update domains) to indicate groups of virtual machines and underlying physical hardware that can be rebooted at the same time. When more than five virtual machines are configured within a single availability set, the sixth virtual machine is placed into the same update domain as the first virtual machine, the seventh in the same update domain as the second virtual machine, and so on. The order of update domains being rebooted may not proceed sequentially during planned maintenance, but only one update domain is rebooted at a time. A rebooted update domain is given 30 minutes to recover before maintenance is initiated on a different update domain.

Fault domains define the group of virtual machines that share a common power source and network switch. By default, the virtual machines configured within your availability set are separated across up to three fault domains for Resource Manager deployments (two fault domains for Classic). While placing your virtual machines into an availability set does not protect your application from operating system or application-specific failures, it does limit the impact of potential physical hardware failures, network outages, or power interruptions.

For more details, refer this documentation.