Q. I understand to receive the full SLA for Azure IaaS VMs I need at least two instances in an Availability Set. Do both have to be running? Do I pay for both?
A. In order to receive the full Azure SLA for IaaS VMs you must have at least two VM instances in an Availability Set. An Availability Set ensures the VMs in the set are distributed over multiple fault domains (think racks) providing protection from unplanned failures and also over multiple update domains to provide availability during planned maintenance. At least two instances must be running to ensure if events happen instances are running to ensure continuation of service availability. Additionally all running instances are billed.