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.

Microsoft's DFS is referred to interchangeably as 'DFS' and 'Dfs' by Microsoft and is unrelated to the DCE Distributed File System, which held the 'DFS' trademark but was discontinued in 2005.

It is called "SMB/CIFS" in some contexts, such as the Open Source OpenSolaris kernel space project. It is also called "MS-DFS" or "MSDFS" in other contexts, e.g. in the Samba user space project.

Overview
DFS has two major logical components. Firstly, DFS namespaces provide an abstraction layer for SMB network file shares, allowing one logical network path to be served by multiple physical file servers. Secondly, DFS supports the replication of data between the servers, using File Replication Service (FRS) in server versions up to Server 2003, and using "DFS Replication" (DFSR) in Server 2003 R2, Server 2008, and later versions.

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 Datacenter 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 roots 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 somewhat rarely encountered because of their limited utility.
 * Domain-based DFS roots exist within Active Directory and can have their information distributed to other domain controllers within the domain — this provides fault tolerance to DFS. DFS roots that exist on a domain must be hosted on a domain controller. This is to ensure that links with the same target get all their information replicated over the network.

DFS Namespaces
Traditional file shares, associated with a single server, have SMB paths of the form \\\ \. Domain-based DFS file share paths are distinguished by using the domain name in place of the server name, in the form \\\ \. When a user access 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 behavior 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 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, by using data compression to reduce network traffic, and by allowing administrators flexible configuration options for limiting network traffic with a weekly schedule.

History
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.