Microsoft Distributed File System (DFS)

Like NFS, DFS employs a tree structure for file systems. A directory that is being imported by a client is attached at a point somewhere in the local file system, where it is then made available to applications as if the directory and its files were local.

The Windows Server family (both 2000 and 2003 versions) includes DFS as an integral part of the operating system. The interface is written as a snap-in to the Microsoft Management Console (MMC) tool, making administration a simpler process. A wizard prompts you through setting up a DFS root, and from there on out you can add, modify, or remove directory paths from the DFS tree. Paths represented in the DFS tree can come from one or more servers on the network. A tree is not bound by a single host.

In Windows NT 4.0, directory replication allowed you to create copies of directories on multiple systems, keeping them in sync. This functionality was replaced in Windows 2000 Server with Distributed File System (though it is also possible to download DFS for Windows NT 4.0). DFS is now included as a part of modern Windows operating systems. DFS allows you to move away from the \\server\ sharename concept to one based on the domain. Instead of having to remember (or browse and find) on which server a particular file share is hosted, you can use DFS to create shares that are global to the domain. That is, if you're a member of the domain, you can specify the share as \\domainname\ share. DFS also allows for replication. This means that you can have more than one copy of the data being shared, but only have to use the global domain-wide share name to connect. This provides for some fault tolerance. If a server goes down that happens to host a replica of a share, then the other servers that contain copies of the share can be connected to by users. Note that this does not provide any kind of failover if a server crashes. The user can, however, restart her work by reconnecting to the share. The connection will be made to another replica of the file share.

Tip

Although DFS does not come with Windows NT 4.0 Server, you can obtain it from Microsoft (www.microsoft.com). The downloadable version can be installed on Microsoft Windows NT Server 4.0 systems and can be used to provide a service to clients similar to that offered by NFS. At the main Web site, click on Downloads and look for it under Downloads for Windows NT 4.0.

Important terms to understand when dealing with DFS are listed here:

• Root A server can have only one root installed. It is just about the same thing as a file share, with a few differences, which will become apparent shortly.

• DFS link Under the root you create DFS links. These are links to file shares that will be available under the root you have created. This means you can create a single root yet place multiple shared directories under the root so that only one file share connection is necessary. If not, users would have to connect to each directory as a separate file share.

• Replica You can create shares that replicate the data in other shares. This can be done to provide for both load balancing and a degree of fault tolerance.

Creating a DFS Root

Creating a DFS tree is not a complicated task when using a graphical interface such as the one provided by the MMC and by the wizards that walk you through setting up your file system. To create a new DFS root, use the following steps:

1.	Click Start, Programs (All Programs for Windows Server 2003), Administrative Tools, Distributed File System.
2.	When the management console appears, select New Root from the Action menu. A wizard pops up. Click Next.
3.	You can select to create a file system that makes use of the Active Directory for storing the DFS configuration information. You can also select to not use the Active Directory. For the purposes of this example, I've chosen to not use the Active Directory. After making the selection, click Next.
4.	The next dialog box prompts you to enter the name of the server that will host the distributed file system. The default is usually the server on which you are running the wizard. Use the default or enter another server. Click Next to continue.
5.	The wizard next prompts you to enter the name of the new root DFS file system. Note that what you enter here shows up in the field Share to Be Used at the bottom of the dialog box. You can use the name of an existing share in the Root Name field, or create the share to be used after you complete this wizard.
6.	If you entered a share that does not yet exist, you can now choose the folder to share by entering it in the field or by using the Browse button.
7.	Finally, the wizard displays a summary of the information you have entered. Click the Finish button to complete the process of creating the new DFS root.

When you're finished, your DFS root will show up in the tree structure in the left pane of the MMC, as you can see in Figure 31.5.

Figure 31.5. The new DFS root is now displayed in the MMC.

Adding Links to the DFS Root

After you've created the initial root for your file system, you can add one or more links. Links, as you will recall, are actual directories (or folders, depending on which terminology you prefer) that will be accessible from your DFS root.

To create a link, right-click on the root in the left pane of the console. From the menu, select New DFS Link. In Figure 31.6 you can see the dialog box used to input the information about the link.

Figure 31.6. You can specify a logical name that users will see so that they don't have to remember the actual directory path.

First you need to enter a name that will be used as a logical association for the link. That is, when the user connects to the share, this name will show up as a folder he can use. You can see that in the next field you are asked to fill in the actual pathname for the folder being offered as part of the DFS share.

In the example in Figure 31.6, the folder that users will see is called accountspayable. The shared directory that contains the actual files is \\Njnet1\wmpub. Note that there is also a comment field in this dialog box. In a large network with many resources, you'll appreciate this information when managing your network. Finally, the last field allows you to specify how long a client will cache the link. When the client makes the initial connection, the connection will be made to the actual directory. However, the user's computer will check back with the DFS server when the cache timer expires.

DFS is a domain-wide file system. In the dialog box shown in Figure 31.6, you can easily browse a file share located on another server in the domain. Thus, you can hide the physical location of files from users. Instead of having to remember servers and share names, the user only has to remember the share name. Because the share name is domain-wide, you can't use the same share name on one server that you do on another. Users can then simply connect to the share \\domainname\share. They don't need to know the server names on which the actual folders in this share are located.

You can use the Action menu to add or delete a DFS root. You can also use the Action menu to manage links and replication.

DFS does not add any additional security features to the file system. Instead, the usual rights and permissions that are already in place on the server are used when evaluating a client's access to a file or directory in the DFS tree.

One major difference between NFS and DFS is that DFS is built using SMB messaging techniques for the most part and is not compatible with all NFS servers. If you have a mixed-environment network in which most of your data files are offered via NFS on Unix servers, it would be more economical to acquire PC-based NFS client software than it would be to replace all your existing servers with NT DFS servers.