Number of Update Domains to use in Availability Set

Q. When creating an Availability Set in Azure the default is 5 update domains but I can set up to 20. What number should I use?

A. Availability Sets provide protection from planned and unplanned downtime. Unplanned protection is provided by distributing VMs in an Availability Set across three fault domains (which can be though of as racks). Planned protection (for example during Azure fabric host updates) is through update domains by only updating one update domain at a time in an Availability Set. This means if workloads are spread over 5 update domains then during planned maintenance one fifth (20%) will be unavailable. This is typically a satisfactory reduction in scale in smaller deployments however for large deployments losing 20% may be unacceptable which is why increasing the number of update domains is performed. The higher the number of update domains the smaller the number of instances that are unavailable during each planned maintenance cycle. The only downside of increasing is the total time of the update will take longer however since the service is still available with a reduced reduction in scale this is likely not a major consideration.