what is fsmo roles
In a single-master model, only one DC in the entire directory is allowed to process updates. This is similar to the role given to a primary domain controller (PDC) in earlier versions of Windows (such as Microsoft Windows NT 4.0), in which the PDC is responsible for processing all updates in a given domain. In a forest, there are five FSMO roles that are assigned to one or more domain controllers. The schema master domain controller controls all updates and modifications to the schema. Once the Schema update is complete, it is replicated from the schema master to all other DCs in the directory.