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We have a service with low SLA requirements, so we host it on a single VM, no need for multiple VMs in availability set/zones. What happens if there is a zone or fault domain failure? Will Azure automatically reallocate the VM to an operational zone / host (FD), or we have to actively restart or redeploy the VM in order to reallocate it?

Setec Astronomy
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From the document, when facing an unexpected Downtime, Azure will migrate your VM to a healthy physical machine in the same datacenter.

When detected, the Azure platform automatically migrates (heals) your virtual machine to a healthy physical machine in the same datacenter. During the healing procedure, virtual machines experience downtime (reboot) and in some cases loss of the temporary drive. The attached OS and data disks are always preserved.

However, if you are using a single VM, it's recommended to use Standard SSD wither higher SLA.

A single instance virtual machine with a Standard SSD will have an SLA of at least 99.5%, while a single instance virtual machine with a Standard HDD will have an SLA of at least 95%. See SLA for Virtual Machines.

Nancy
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    Thanks, per this document I now understand that Azure will move the VM to a healthy host on the same DC. I wonder what happens in case of a DC / Zone failure, again for a single VM deployment. will the VM be moved to a new zone or just left on faulty zone? – Setec Astronomy Nov 09 '20 at 06:11
  • Per my understanding. For a single VM without availability set/zones. By default, it has no infrastructure redundancy required(you can check this when you deploy VM via Azure portal). In case of n case of a DC / Zone failure, Azure guarantees for Single Instance Virtual Machines based on Storage type. That is [What Azure Availability Zones Are and Why You Should Use Them](https://dzone.com/articles/what-are-azure-availability-zones-and-why-should-y) – Nancy Nov 11 '20 at 09:53