Distributed File System
Updated: July 15, 2009
Applies To: Windows Server 2008
Distributed File System (DFS) Namespaces and DFS Replication offer simplified, highly-available access to files, load sharing, and WAN-friendly replication. In the Windows Server® 2003 R2 operating system, Microsoft revised and renamed DFS Namespaces (formerly called DFS), replaced the Distributed File System snap-in with the DFS Management snap-in, and introduced the new DFS Replication feature. In the Windows Server® 2008 operating system, Microsoft added the Windows Server 2008 mode of domain-based namespaces and added a number of usability and performance improvements.
What does Distributed File System (DFS) do?
The Distributed File System (DFS) technologies offer wide area network (WAN)-friendly replication as well as simplified, highly-available access to geographically dispersed files. The two technologies in DFS are the following:
DFS Namespaces. Enables you to group shared folders that are located on different servers into one or more logically structured namespaces. Each namespace appears to users as a single shared folder with a series of subfolders. This structure increases availability and automatically connects users to shared folders in the same Active Directory Domain Services site, when available, instead of routing them over WAN connections.
DFS Replication. DFS Replication is an efficient, multiple-master replication engine that you can use to keep folders synchronized between servers across limited bandwidth network connections. It replaces the File Replication Service (FRS) as the replication engine for DFS Namespaces, as well as for replicating the AD DS SYSVOL folder in domains that use the Windows Server 2008 domain functional level.
Who will be interested in this feature?
Administrators of large networks who want to organize and increase the availability of shared folders by creating a namespace and administrators who want to keep folders synchronized between servers in an efficient manner by using DFS Replication will be interested in this feature.
What new functionality does this feature provide?
DFS in Windows Server 2008 is implemented as a role service of the File Services role. The Distributed File System role service consists of two child role services:
To manage DFS Namespaces and DFS Replication on a computer running Windows Server 2008, you can use the DFS Management snap-in hosted by Server Manager, or you can use the DFS Management snap-in from the Administrative Tools folder.
The following sections describe the individual changes in DFS Namespaces and DFS Replication for Windows Server 2008.
What new functionality is provided by DFS Namespaces?
DFS Namespaces in Windows Server 2008 includes the following changes.
Access-based enumeration allows users to see only files and folders on a file server to which they have permission to access. This feature is not enabled by default for namespaces (though it is enabled by default on newly-created shared folders in Windows Server 2008), and is only supported in a DFS namespace when the namespace is a standalone namespace hosted on a computer running Windows Server 2008, or a domain-based namespace by using the Windows Server 2008 mode.
To enable access-based enumeration in a namespace, see Enable Access-Based Enumeration on a Namespace.
DFS Namespaces in Windows Server 2008 supports creating stand-alone namespaces on a failover cluster from within the DFS Management snap-in. To do so, specify a failover cluster on the Namespace Server page of the New Namespace Wizard.
DFS Replication service is not designed to coordinate with cluster components, and the service will not fail over to another node.
Improved command-line tools
DFS Namespaces in Windows Server 2008 includes an updated version of the Dfsutil command and the new Dfsdiag command, which you can use to diagnose namespace issues.
For more information about the changes to Dfsutil, see (https://go.microsoft.com/fwlink/?LinkId=136572).
For more information about Dfsdiag, see (https://go.microsoft.com/fwlink/?LinkId=136571).
Search for folders or folder targets within a namespace
DFS Management in Windows Server 2008 includes the ability to search for folders or folder targets within a namespace. To use this feature, select a namespace, click the Search tab, type your search string in the text box, and then click Search.
Windows Server 2008 mode domain-based namespaces
Windows Server 2008 includes the ability to create a domain-based namespace in Windows Server 2008 mode. Doing so enables support for access-based enumeration and increased scalability. The domain-based namespace introduced in Windows® 2000 Server is now referred to as "domain-based namespace (Windows 2000 Server mode)."
To use the Windows Server 2008 mode, the domain and domain-based namespace must meet the following minimum requirements:
The forest uses the Windows Server 2003 or higher forest functional level.
The domain uses the Windows Server 2008 or higher domain functional level.
All namespace servers are running Windows Server 2008.
If your environment supports it, choose the Windows Server 2008 mode when you create new domain-based namespaces. This mode provides additional features and scalability, and also eliminates the possible need to migrate a namespace from the Windows 2000 Server mode.
For information about migrating a namespace to Windows Server 2008 mode, see Migrate a Domain-based Namespace to Windows Server 2008 Mode.
What new functionality is provided by DFS Replication?
DFS Replication in Windows Server 2008 includes the following changes.
DFS Replication in Windows Server 2008 has a new feature called Content Freshness, which prevents a server that was offline for a long time from over-writing fresh data when it comes back online with stale (out-of-date) data.
Improvements for handling unexpected shutdowns
In Windows Server 2008, DFS Replication now allows for quicker recovery from unexpected shutdowns. Unexpected shutdowns can occur because of the following reasons:
Unexpected shutdown of DFS Replication: This could occur if the DFS Replication process crashes, is ended, or stops because there are insufficient resources.
Unexpected shutdown of the computer: This could occur if the computer crashes or loses power while DFS Replication is running.
Unexpected shutdown of the volume: This could occur if the volume hosting a DFS Replication content set loses power, is disconnected, or is forced to dismount.
Unexpected shutdowns of the computer and the volume can cause the NTFS file system to lose changes which have not been copied to disk. Therefore the DFS Replication database can become inconsistent with the on-disk file system state.
On Windows Server 2003 R2, an unexpected shutdown may force DFS Replication to perform a complete database rebuild, which can be very time consuming. DFS Replication in Windows Server 2008 usually does not need to rebuild the database following unexpected shutdowns, and thus recovers much more quickly.
These improvements are available only if all members of the replication group are running Windows Server 2008.
DFS Replication performance improvements
DFS Replication in Windows Server 2008 includes the following performance improvements:
Faster replication both for small and large files.
Initial synchronization completes faster.
Better network bandwidth utilization on LANs and high latency networks such as WANs.
The following table provides additional details about the performance improvements in DFS Replication.
|Windows Server 2003 R2
|Windows Server 2008
Multiple RPC calls
RPC Async Pipes (when replicating with other servers running Windows Server 2008)
Synchronous inputs/outputs (I/Os)
Normal Priority I/Os
Low Priority I/Os (this reduces the load on the system as a result of replication)
4 concurrent file downloads
16 concurrent file downloads
DFS Management in Windows Server 2008 includes a new type of diagnostic report called a propagation report. This report displays the replication progress for the test file created during a propagation test.
DFS Management now includes the ability to force replication to occur immediately, temporarily ignoring the replication schedule.
To force replication immediately
In the console tree, under the Replication node, select the appropriate replication group.
Click the Connections tab.
Right-click the member you want to use to replicate, and then click Replicate Now.
Support for Read-Only Domain Controllers
In Windows Server 2008, DFS Replication supports Read-Only Domain Controllers (RODCs). For more information about RODCs, see https://go.microsoft.com/fwlink/?LinkId=96517.
On an RODC, any changes made to the domain controller are rolled back by DFS Replication.
DFS Replication does not support read-only replication groups other than the SYSVOL folder on domain controllers, and only supports RODCs in leaf nodes.
SYSVOL replication using DFS Replication
DFS Replication replaces the File Replication Service (FRS) as the replication engine for replicating the AD DS SYSVOL folder in domains that use the Windows Server 2008 domain functional level.
To facilitate migrating existing SYSVOL folders to DFS Replication, Windows Server 2008 includes a tool that helps to migrate the replication of existing SYSVOL folders from FRS to DFS Replication. This tool:
Enables administrators to initiate the migration of SYSVOL folders to the DFS Replication service by specifying all required options and has intelligent predefined defaults.
Provides mechanisms for administrators to troubleshoot potential problems that could occur during migration.
Has monitoring capabilities that enable administrators to view the progress of the migration process.
The results of using the Dcpromo tool on a computer running Windows Server 2008 vary depending on the domain functional level:
If the domain functional level is Windows Server 2008, the server will use DFS Replication for SYSVOL replication.
If the domain functional level is Windows Server 2003, the server will use FRS for SYSVOL replication.
For more information about replicating SYSVOL using DFS Replication, see (https://go.microsoft.com/fwlink/?LinkId=93057).
To manage a Distributed File System namespace that uses FRS to replicate content, open the Distributed File System snap-in on a computer running Windows Server 2003 or Windows 2000 Server. The only FRS management operations that DFS Management in Windows Server 2008 can perform are displaying replica sets and deleting them.