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Flexible Single Master Operations (FSMO, F is sometimes floating ; pronounced Fiz-mo), or just single master operation or operations master, is a feature of Microsoft's Active Directory (AD). As of 2005, the term FSMO has been deprecated in favour of operations masters.
FSMO is a specialized domain controller (DC) set of tasks, used where standard data transfer and update methods are inadequate. AD normally relies on multiple peer DCs, each with a copy of the AD database, being synchronized by multi-master replication. The tasks which are not suited to multi-master replication, and are viable only with a single-master database, are the FSMOs.
These roles are applicable at the domain level
These roles are unique at enterprise level
To summarize then, the Schema Master and Domain Naming Master roles are found only in the forest root domain, while the remaining roles are found in each domain of your forest.
By default AD assigns all operations master roles to the first DC created in a forest. If new domains are created in the forest, the first DC in a new domain holds all of the domain-wide FSMO roles. This is not a satisfactory position. Microsoft recommends the careful division of FSMO roles, with standby DCs ready to take over each role. When a FSMO role is transferred to a different DC, the original FSMO holder and the new FSMO holder communicate to ensure no data is lost during the transfer. If the original FSMO holder experienced an unrecoverable failure, you can force another DC to seize the lost roles; however, there is a risk of data loss because of the lack of communications. If you seize a FSMO role instead of transferring the role, that domain controller can never be allowed to host that FSMO role again, except for the PDC emulator Master operation and the Infrastructure Master Operation. Corruption can occur within Active Directory. FSMO roles can be easily moved between DCs using the AD snap-ins to the MMC or using
ntdsutil which is a command line based tool.
Certain FSMO roles depend on the DC being a Global Catalog (GC) server as well. For example, the Infrastructure Master role must not be housed on a domain controller which also houses a copy of the global catalog in a multi-domain forest (unless all domain controllers in the domain are also global catalog servers), while the Domain Naming Master role should be housed on a DC which is also a GC. When a Forest is initially created, the first Domain Controller is a Global Catalog server by default. The Global Catalog provides several functions. The GC stores object data information, manages queries of these data objects and their attributes as well as provides data to allow network logon.
The PDC emulator and the RID master should be on the same DC, if possible. The Schema Master and Domain Naming Master should also be on the same DC. To provide fault tolerance, there should be at least 2 domain controllers available within each domain of the Forest. Furthermore, the Infrastructure Master role holder should not also be a Global Catalog Server, as the combination of these two roles on the same host will cause unexpected (and potentially damaging) behaviour in a multi-domain environment.
To prevent conflicting updates in Windows 2000, the Active Directory performs updates to certain objects in a single-master fashion. [...] Because an Active Directory role is not bound to a single DC, it is referred to as a Flexible Single Master Operation (FSMO) role.