Windows® Internals, Sixth Edition, Part 1

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.

Offline Files

Offline Files (also known internally as client-side caching, or CSC) transparently caches files from a remote system (a file server) on the local machine to make the files available when the local machine is not connected to the network. Offline Files caches files for remote files accessed over the SMB protocol. Files can be cached by users by simply right-clicking on a remote file, folder, or drive and selecting Always Available Offline, thus pinning the selected files to the cache. Cached items can be viewed in the Sync Center control panel. Caching also can be specified administratively using Group Policy.

There is a single Offline Files cache on the system, which is shared by all users of the system. All cached files are stored in an ACL-protected directory, which by default is %SystemRoot%\CSC. If you choose, you can encrypt the files in the Offline Files cache (accessed by going to Control Panel, Sync Center, and then clicking Manage Offline Files, clicking on the Encryption tab, and clicking the Encrypt button). Access to the cache is permitted only by using Offline File tools and the IOfflineFilesXxx COM APIs. The easiest way to examine the contents of the cache is to use the Sync Center control panel interface (click Manage Offline Files, and then click the View Your Offline Files button).

Offline Files understands two types of objects:

Files. Includes files, folders, and symbolic links. Caching is not done at the NTFS level, so not all file NTFS attributes are cached or are cacheable. Cacheable attributes include the standard Win32 file attributes (metadata), such as the name, ACL, and the contents—only a file’s (unnamed) data stream will be cached.

Scope. A scope is the portion of a namespace that corresponds to a physical share. In a DFS namespace, the root of a scope is the object that is pointed to by a DFS link, which can contain additional DFS links to other scopes. If DFS is not being used, a scope and a share are the same thing.

Offline Files does not support complete NTFS semantics for cached files and has the following limitations:

Offline Files does not cache alternate data streams, which are therefore not available offline. When online, access to alternate data streams works because I/O requests for streams go directly to the server.

Offline Files does not cache Extended Attributes (EAs). An implication of this is that if a file containing EAs is cached and the cached version is modified while the server is offline, any EAs on the server are deleted when changes are written back to the server.