The question of availability is a key issue for the provisioning of virtual machines in Azure. It is necessary to understand how datacenters are structured and where VMs are positioned in order to shed additional light on this aspect.
Fault and update domains have an important role to play in the operation of virtual machines in Azure. Here, fault domains describe systems with identical dependencies such as power supply, network access or server racks. Update domains are systems in which the same maintenance cycle – for instance identical patching – is applied. There are several fault and update domains in each Azure region.
Please bear in mind that systems running within the same fault and/or update domain may under certain circumstances experience simultaneous outage!
This is why SLAs are not provided in any form for individual VMs running in Azure . At least two virtual machines must be operated within one availability group in order to receive guaranteed availability. In this respect, the system will be set up to ensure that VMs offering the same service are operated on hosts with varying maintenance windows and in different DC regions. This way there will always be at least one virtual machine that remains up and running within the availability group.