Most domain controller functionality in Windows Server 2000, Windows Server 2003, Windows Server 2008, and Windows Server 2008 R2 was designed to be distributed, multimaster-based. This effectively eliminated the single point of failure that was present with Windows NT primary domain controllers (PDCs). However, five functions still require the use of a single server because their functionality makes it impossible to follow a distributed approach. These Operations Master (OM) roles (previously referred to as FSMO roles) are outlined as follows:
. Schema master—There is only one writable master copy of the AD DS schema in a single AD DS forest. It was deliberately designed this way to limit access to the schema and to minimize potential replication conflicts. There can be only one schema master in the entire AD DS forest.
. Domain naming master—The domain naming master is responsible for the addition of domains into the AD DS forest. This OM role must be placed on a global catalog server because it must have a record of all domains and objects to perform its function. There can be only one domain naming master in a forest.
. PDC emulator—This role used to exist to emulate the legacy Windows NT 4.0 primary domain controller (PDC) for down-level clients. With Windows Server 2008 R2, the PDC emulator still performs certain roles, such as acting as the primary time sync server for the domain. There is one PDC emulator FSMO role per AD DS domain.
. RID master—All objects within AD DS that can be assigned permissions are uniquely identified through the use of a security identifier (SID). Each SID is composed of a domain SID, which is the same for each object in a single domain, and a relative identifier (RID), which is unique for each object within that domain. When assigning SIDs, a domain controller must be able to assign a corresponding RID from a pool that it obtains from the RID master. When that pool is exhausted, it requests another pool from the RID master. If the RID master is down, you might not be able to create new objects in your domain if a specific domain controller runs out of its allocated pool of RIDs. There is one RID master per AD DS domain.
. Infrastructure master—The infrastructure master manages references to domain objects not within its own domain. In other words, a DC in one domain contains a list of all objects within its own domain, plus a list of references to other objects in other domains in the forest. If a referenced object changes, the infrastructure master handles this change. Because it deals with only referenced objects and not copies of the object itself, the infrastructure master must not reside on a global catalog server in multiple domain environments. The only exceptions to this are if every domain controller in your domain is a global catalog server or if you are in a single-domain environment. In the first case, there is no need to reference objects in other domains because full copies are available. In the second case, the infrastructure master role is not utilized because all copies of objects are local to the domain.
Transfer of an OM role to another domain controller can be performed as part of regular maintenance, or in the case of a disaster recovery situation where an OM server is brought offline, the OM can be seized to be brought back online. This is true for conditions where the schema master, domain naming master, PDC emulator, infrastructure master, or RID master either needs to be moved to another system (transfer) or has gone down and no backup is available (seized). The transfer and seizure of an OM role is done through the use of a command-line tool called ntdsutil, shown in Figure 4.4. Keep in mind that you should use this utility only in emergency situations and should never bring the old OM server that has had its role seized back online into the domain at risk of some serious system conflicts.
Source of Information : Sams - Windows Server 2008 R2 Unleashed