Understanding virtual machine availability
A common misconception in Azure is that VMs are automatically highly available. Although this may be true to a certain extent as the failure of hardware results in a VM being moved to healthy hardware, this process temporarily interrupts the accessibility of that VM.
Additionally, during maintenance events, the Azure platform may need to forcefully reboot your VM. This is performed gracefully, but again it causes a brief outage for your workload.
Finally, in the unlikely event of an entire region outage, for example, due to networking failure, your VMs will be inaccessible until that outage is rectified.
Another aspect of your VM availability is the type of disks you choose to build it with. Standard magnetic HDDs have the lowest availability, whereas premium SSDs have the greatest due to how they are used and distributed.
These factors can have a significant impact on the Service-Level Agreement (SLA) of your service. For example...