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.
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