Windows® Internals, Sixth Edition, Part 1

Distributed File System Namespace (DFS-N) is a namespace aggregation and availability feature of Windows. As organizations grow, the number of file servers tends to increase, and users find it increasingly difficult to find the files they need because the files might be spread over a number of different servers with completely unrelated names. DFS-N allows an administrator to create a new file share (also known as a root or namespace) that aggregates multiple file shares, from the same or different servers, into a single namespace. For example, assume the Aura Corporation had the following shares: \\Development\Projects, \\Accounting\FY2012, and \\Marketing\CoolStuff. These shares could be presented to users through a DFS-N namespace \\Aura\Teams containing DFS-N links called \\Aura\Teams\\Aura\Development, \\Aura\Teams\Accounting, and \\Aura\Teams\Marketing. The redirection of a client accessing the path \\Aura\Teams\Marketing to the real share path \\Marketing\CoolStuff is invisible to the user. In this example, \\Marketing\CoolStuff is the link target of \\Aura\Teams\Marketing. Link targets can, in fact, refer to paths below the root of a share like \\Marketing\CoolStuff\Presentations.

Other benefits that DFS-N provides are redundancy and location-aware redirection. Another major capability of DFS is availability, through a feature known as DFS Replication (DFSR). Replication provides two benefits: high availability in case of a failure, and load balancing. As an organization grows geographically, accessing file servers from remote offices with wide area network (WAN) connections might be slow and inefficient. An administrator could create a replicated version of a file server within the remote office, providing high-speed access to the files from the users within the remote office. A DFS-N link, such as \\Aura\Teams\Accounting in the preceding example, might have multiple link targets associated with it—for example, \\AccountingEurope\FY2012 and \\AccountingUS\FY2012. In this case, the DFS-N server returns to the client an ordered list of available target servers and takes into account the location of the client and the target servers (using Active Directory site information) when ordering the list so that the client can access the closest target first. If access to one link target fails, DFS-N tries the next available target, if available. When a DFS-N link has multiple target shares, the targets should normally contain the same data because the client accessing the namespace will access only one of the targets at a time. This can be accomplished using DFS Replication (DFS-R), discussed in the next section. A server-side implementation of DFS-N consists of a Windows service (%SystemRoot%\System32\Dfssvc.exe) and a device driver (%SystemRoot%\System32\Drivers\Dfs.sys). The DFSSVC service is responsible for exporting DFS topology-management interfaces and maintaining the DFS topology in either the registry (on non–Active Directory systems) or Active Directory. The DFS driver performs topology lookups when it receives a client request touching a link so that it can direct the client to the share where the file it is requesting resides.

On the client side, DFS-N support is implemented in a MUP surrogate provider driver (%SystemRoot%\System32\Drivers\Dfsc.sys) and an MPR/WNet provider implemented in %SystemRoot%\System32\Ntlanman.dll. The Distributed File System Client (DFSC) driver is responsible for determining if a UNC path is a DFS namespace, and if so, it translates the specified path into the name of one or more target shares. Communication with DFS-N servers is accomplished using the SMB redirector. The DFS-N client is only part of the I/O path when a file or directory is being created or opened. Once it returns the name of a target share to MUP, DFSC is not involved with subsequent I/O to the file.

The DFS-N protocols are documented in the MS-DFSC and MS-DFSNM protocol documents.

Distributed File System Replication

Distributed File System Replication (DFS-R) provides bandwidth-efficient, asynchronous, multimaster replication of file-system changes between servers. In addition to general-purpose, file-system replication (for example, keeping data on multiple DFS-N link target shares in sync), DFS-R is also used for replicating a domain controller’s \SYSVOL directory, which is where Windows domain controllers store logon scripts and Group Policy files. (Group Policy permits administrators to define usage and security policies for the computers that belong to a domain.) Because DFS-R supports multimaster replication, file-system changes can occur on any server, potentially simultaneously, and DFS-R will automatically handle conflicts and maintain synchronization of the file-system contents.