The MDS is the root of the D3 file system. It contains pointers to the databases. It is located on a special server on the network and designated as the Network Controller. Normally, the network controller would be the Domain Controller. The D3 FSI Server running on the network controller acts as a file server for both the MDS and the ordinary accounts and files located on this system.
The MDS also contains special items (such as the default logon banner).
In practice, there should be only one MDS per domain. However, there are ways to create another MDS and access this file system from C++.
See Also
D3 File System Interface Server Configuration