Distributed File System (Microsoft)
Encyclopedia
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 also called "MS-DFS" or "MSDFS" in some contexts, e.g. in the Samba
user space
project.
for SMB
network file shares, allowing one logical network path to be served by multiple physical file servers. Second, 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:
\\. 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 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 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
.
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.
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.http://www.thursby.com/Mac_OSX_DFS
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
servers to organize many distributed SMB
Server Message Block
In computer networking, Server Message Block , also known as Common Internet File System operates as an application-layer network protocol mainly used to provide shared access to files, printers, serial ports, and miscellaneous communications between nodes on a network. It also provides an...
file shares into a distributed file system
Distributed file system
Network file system may refer to:* A distributed file system, which is accessed over a computer network* Network File System , a specific brand of 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
DCE Distributed File System
The DCE Distributed File System is the remote file access protocol used with the Distributed Computing Environment. It was based on the AFS Version 3.0 protocol that was developed commercially by Transarc Corporation...
, which held the 'DFS' trademark but was discontinued in 2005.
It is also called "MS-DFS" or "MSDFS" in some contexts, e.g. in the Samba
Samba (software)
Samba is a free software re-implementation, originally developed by Andrew Tridgell, of the SMB/CIFS networking protocol. As of version 3, Samba provides file and print services for various Microsoft Windows clients and can integrate with a Windows Server domain, either as a Primary Domain...
user space
User space
A conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers...
project.
Overview
DFS has two major logical components. First, DFS namespaces provide an abstraction layerAbstraction layer
An abstraction layer is a way of hiding the implementation details of a particular set of functionality...
for SMB
Server Message Block
In computer networking, Server Message Block , also known as Common Internet File System operates as an application-layer network protocol mainly used to provide shared access to files, printers, serial ports, and miscellaneous communications between nodes on a network. It also provides an...
network file shares, allowing one logical network path to be served by multiple physical file servers. Second, DFS supports the replication of data between the servers, using File Replication Service
File Replication Service
File Replication Service is a Microsoft Windows Server service for distributing shared files and Group Policy Objects. It replaced the Lan Manager Replication service , and has been partially replaced by Distributed File System Replication...
(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
Windows NT 4.0
Windows NT 4.0 is a preemptive, graphical and business-oriented operating system designed to work with either uniprocessor or symmetric multi-processor computers. It was the next release of Microsoft's Windows NT line of operating systems and was released to manufacturing on 31 July 1996...
and up) and OpenSolaris
OpenSolaris
OpenSolaris was an open source computer operating system based on Solaris created by Sun Microsystems. It was also the name of the project initiated by Sun to build a developer and user community around the software...
(in kernel space) or a computer running Samba
Samba (software)
Samba is a free software re-implementation, originally developed by Andrew Tridgell, of the SMB/CIFS networking protocol. As of version 3, Samba provides file and print services for various Microsoft Windows clients and can integrate with a Windows Server domain, either as a Primary Domain...
(in user space
User space
A conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers...
.) 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 namespace allow for a DFS root that exists only on the local computer, and thus does not use Active DirectoryActive DirectoryActive Directory is a directory service created by Microsoft for Windows domain networks. It is included in most Windows Server operating systems. Server computers on which Active Directory is running are called domain controllers....
. 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\
or \\fq.domain.name\ . 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 \\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
Failover
In computing, failover is automatic switching to a redundant or standby computer server, system, or network upon the failure or abnormal termination of the previously active application, server, system, or network...
.
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 (remote differential compression
Remote Differential Compression
Remote Differential Compression is a client–server synchronization algorithm that allows the contents of two files to be synchronized by communicating only the differences between them...
), by using data compression to reduce network traffic, and by allowing administrators flexible configuration options for limiting network traffic with a customizable schedule.
History
The server component of Distributed File System was first introduced as an add-on to Windows NT 4.0Windows NT 4.0
Windows NT 4.0 is a preemptive, graphical and business-oriented operating system designed to work with either uniprocessor or symmetric multi-processor computers. It was the next release of Microsoft's Windows NT line of operating systems and was released to manufacturing on 31 July 1996...
Server, called "DFS 4.1", and was later included as a standard component of all editions of Windows 2000
Windows 2000
Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...
Server. Windows NT 4.0 and later include client-side support for DFS.
Versions 2.6.14 and later of the Linux kernel
Linux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
come with an SMB client VFS
Virtual file system
A virtual file system or virtual filesystem switch is an abstraction layer on top of a more concrete file system. The purpose of a VFS is to allow client applications to access different types of concrete file systems in a uniform way...
called "cifs" that supports DFS. Thursby Software's DAVE, ADmitMac and ADmitMac PKI products support DFS commercially on Mac OS X.http://www.thursby.com/Mac_OSX_DFS