Update domain

A fault domain takes care of unplanned maintenance and an update domain handles planned maintenance. Each VM is also assigned an update domain. There can be as many as 20 update domains in a single availability set. Update domains are non-configurable by users. When multiple VMs are created, they are placed on separate update domains. If more than 20 VMs are provisioned on an availability set, they are placed in a round-robin fashion on these update domains. Update domains take care of planned maintenance.