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.
The fundamental unit of DFS replication is a DFS replicated folder, which is a directory tree whose contents will be synchronized across multiple servers according to an administratively defined schedule and replication topology. Replication schedules allow administrators to restrict replication activity to specific windows of time or restrict the amount of bandwidth that DFS-R will use.
Replication topologies allow administrators to define the network connections between a set of servers (called a replication group). Arbitrary topologies are supported, including common topologies such as ring, star, or mesh. The replication topology configuration is stored in Active Directory. Only directories on NTFS volumes can be replicated because DFS-R relies on the NTFS USN journal to detect changes to the contents of a replicated folder.
DFS-R uses several technologies to conserve network bandwidth, making it well-suited to replication over WANs that might have high latency and low bandwidth. Remote Differential Compression (RDC) allows DFS-R to identify and replicate only those pieces of a file that have changed, rather than the whole file. DFS-R also compresses content before sending it to a remote partner, providing additional bandwidth savings. On Enterprise or Datacenter SKUs, DFS-R makes use of an extended version of RDC called RDC Similarity to provide further bandwidth savings; if content is modified in a replicated folder on server A, and chunks of the modified content are similar to chunks of any file in partner server B’s replicated folder, server B satisfies the similar chunks of the update’s content locally from the similar files, rather than downloading all of the modified content from server A.
New capabilities for DFS-R in Windows Server 2008 R2 include support for clustering and true read-only replicas.
DFS-R is implemented as a Windows service (%SystemRoot%\System32\DfsrS.exe) that uses authenticated RPC with encryption to communicate between instances of itself running on different computers. There is also a WMI interface for configuration and management of the service, a file system minifilter used to protect read-only replicas from modification, and a cluster resource DLL for integration with MSCS. The DFS-R protocol is documented in the MS-FRS2 specification.