In Active Directory there are five operations master roles known as FSMO roles. This video looks at which Domain Controllers you should put these roles on and also which Domain Controllers you should make into Global Catalog Servers.
There are five operations master roles. The Schema and Domain Naming Masters are forest wide so there will only one of each of these roles regardless of how many domains you have in your forest. The PDC Emulator, RID Master and Infrastructure Master are domain wide. There will always be 3 operations master roles per domain, one of each. When considering where to put the operations master roles, you should consider the availability of the operations role and what effect not having the operations master role available during an outage will have on your network.
Schema Master (Forest wide)
The Schema Master is generally found in the root domain in a multiple domain environment. On most networks it will not be used that often. For this reason availability is not a big issue so for ease of administration it will often be put on the same Domain Controller that has the Domain Naming Master. The Schema Master operations master role is not affected whether the Domain Controller is a Global Catalog Server or not.
Domain Naming Master (Forest wide)
The Domain Naming Master is required when domains are added or removed from the forest. It does require Global Catalog calls when domains are added or removed. For this reason it is recommended to make it a Global Catalog Server. However, this will not affect operations if it is not.
The PDC Emulator has the final say on authentication. For this reason the PDC Emulator will generally be placed on the network with the most users. The PDC Emulator can be made a Global Catalog Server; however, administrators will often remove the Global Catalog from the PDC Emulator if performance on the PDC Emulator becomes a problem.
The RID Master allocates blocks of RIDs. For this reason it does not have to be on the fastest Domain Controller or on the fastest link. Domain Controllers will request RIDs before they run out. The PDC Emulator generally uses more RIDs than other Domain Controllers on the network and thus a lot of administrators will place the RID operations master role on the same Domain Controller that is holding the PDC Emulator. Whether the Domain Controller is a Global Catalog Server or not does not affect the operation of the RID Master.
The Infrastructure Master role tracks references in multi-domain environments. In a single domain network the Infrastructure operations master role is not that important. In a multi-domain environment the role of the Infrastructure Master becomes more important. The choice of whether to make this a Global Catalog Server or not can affect its ability to keep cross domain reference up to date.
If you have Windows Server 2000 or 2003 Domain Controllers on your network, you need to ensure the Infrastructure Master is not a Global Catalog Server or all your Domain Controllers on the network will become Global Catalog Servers. In a pure Windows Server 2008 environment, it does not matter whether you make the Domain Controller a Global Catalog Server or not.
Disadvantages of making a Domain Controller a Global Catalog Server
Making a Domain Controller a Global Catalog Server will increase the amount of hard disk space that it requires and also the amount of network bandwidth that it will use. Nowadays it is not as big of a concern as it was when Windows Server 2000 came out. Global Catalog Servers are also used by clients to perform searches and to look up objects. This can increase the load on the Domain Controller.