Distributed File System (Microsoft)
Distributed File System (DFS) is a set of client and server services that allow an organization using Microsoft Windows servers to organize many distributed SMB file shares into a distributed file system. DFS provides location transparency and redundancy to improve data availability in the face of failure or heavy load by allowing shares in multiple different locations to be logically grouped under one folder, or DFS root.
There is no requirement to use the two components of DFS together; it is perfectly possible to use the logical namespace component without using DFS file replication, and it is perfectly possible to use file replication between servers without combining them into one namespace.
A DFS root can only exist on a server version of Windows (from Windows NT 4.0 and up) and OpenSolaris (in kernel space) or a computer running Samba (in user space.) The Enterprise and Data center Editions of Windows Server can host multiple DFS roots on the same server. OpenSolaris intends on supporting multiple DFS roots in "a future project based on Active Directory (AD) domain-based DFS namespaces".
There are two ways of implementing DFS on a server:
- Standalone DFS namespace allow for a DFS root that exists only on the local computer, and thus does not use Active Directory. A Standalone DFS can only be accessed on the computer on which it is created. It doesn't offer any fault tolerance and cannot be linked to any other DFS. This is the only option available on Windows NT 4.0 Server systems. Standalone DFS roots are rarely encountered because of their limited utility.
- Domain-based DFS namespace stores the DFS configuration within Active Directory, the DFS namespace root is accessible at \\domainname\<dfsroot> or \\fq.domain.name\<dfsroot>. The namespace roots do not have to reside on domain controllers, they can reside on member servers, if domain controllers are not used as the namespace root servers, then multiple member servers should be used to provide full fault tolerance.
DFS namespaces 
Traditional file shares, associated with a single server, have SMB paths of the form \\<SERVER>\<path>\<subpath>. Domain-based DFS file share paths are distinguished by using the domain name in place of the server name, in the form \\<DOMAIN.NAME>\<dfsroot>\<path>. When a user accesses such a share, either directly or by mapping a drive, their computer will access one of the available servers associated with that share, following rules which can be configured by the network administrator. For example, the default behaviour is that users will access the closest server to them; but this can be overridden to prefer a particular server.
If a server fails, the client can select a different server transparently to the user. One major caveat regarding this flexibility is that currently-open files will potentially become unusable, as open files cannot be failed-over.
DFS replication 
Early versions of DFS used Microsoft's File Replication Service (FRS) which provides basic file replication capability between servers. FRS identifies changed or new files, and copies the latest version of the entire file to all servers.
Windows Server 2003 R2 introduced "DFS Replication" (DFSR) which improves on FRS by only copying those parts of files which have changed (remote differential compression), by using data compression to reduce network traffic, and by allowing administrators flexible configuration options for limiting network traffic with a customizable schedule.
The server component of Distributed File System was first introduced as an add-on to Windows NT 4.0 Server, called "DFS 4.1", and was later included as a standard component of all editions of Windows 2000 Server. Windows NT 4.0 and later include client-side support for DFS. Versions 2.6.14 and later of the Linux kernel come with an SMB client VFS called "cifs" that supports DFS. Thursby Software's DAVE, ADmitMac and ADmitMac PKI products support DFS commercially on Mac OS X. DFS is reported to be supported natively in Mac OS X 10.7 ("Lion").
- How DFS Works: Remote File Systems
- [MS-DFSC]: Distributed File System (DFS): Referral Protocol Specification