Azure periodically performs updates to improve the reliability, performance, and security of the host infrastructure for virtual machines. Updates are changes like patching the hosting environment or upgrading and decommissioning hardware. A majority of these updates are performed without any impact to the hosted virtual machines. However, there are cases where updates do have an impact.

How do I know maintenance is happening?

• In the Azure portal you can add the "Maintenance" column
• Scheduled maintenance is reported to Azure Activity Logs so you can create a Health Alert based on that
  • Make sure you set the Event type as Planned maintenance and Services as Virtual Machine Scale Sets and/or Virtual Machines
    

What can I do to minimize downtime?

• If the host does not require a reboot, Azure will pause your VM, patch the host, and then resume the VM
• If the host does require a reboot, you will, typically, get notice 35 days in advance

Why does my VM still show pending maintenance even after a deallocate/restart or redeploy?

• There exists an edge case where after you issue a deallocate/restart or a redeploy your VM may start on a host that is still scheduled for maintenance. This is a known scenario with no good fix other than another deallocate/restart or redeploy.

Links

https://docs.microsoft.com/en-us/azure/virtual-machines/windows/maintenance-notifications

https://docs.microsoft.com/en-us/azure/virtual-machines/windows/maintenance-and-updates

https://docs.microsoft.com/en-us/azure/virtual-machines/windows/scheduled-events

https://docs.microsoft.com/en-us/azure/service-health/alerts-activity-log-service-notifications

https://azure.microsoft.com/en-us/blog/find-out-when-your-virtual-machine-hardware-is-degraded-with-scheduled-events/

https://azure.microsoft.com/en-us/blog/improving-azure-virtual-machine-resiliency-with-predictive-ml-and-live-migration/