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Windows Server Enterprise Administration : Planning for Data Sharing and Collaboration (part 1)

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Distributed file system (DFS) and SharePoint facilitate data sharing in large organizations, but in very different ways. DFS is a feature whose main benefit is to replicate file shares to remote offices and to provide a consistent Universal Naming Convention (UNC) pathname to file shares regardless of location in a network.

SharePoint, on the other hand, provides access to data through team Web sites. SharePoint sites can store files and documents, but they also provide version control, bulletin boards, calendaring, and many other features.


Planning a DFS Deployment

DFS is a feature in Windows Server 2008 that facilitates access to shared files in a large network. As part of your overall network planning for data sharing and collaboration, you should consider your network needs for file sharing, review the features offered by DFS, and then determine whether this feature can meet those needs.

Reviewing DFS Concepts and Features

DFS enables an organization to build a single hierarchical view of file shares that remains consistent across sites in a large network. Users access DFS shares by specifying an alias pathname that remains identical regardless of location. With DFS, shared files are replicated among multiple servers so that by specifying the same pathname, users throughout the network access a local copy of the hosted files. When permissions allow changes to a file or folder, changes made to the local copy are also replicated to other DFS servers.

Important: DFS fundamentals

If you are not familiar with basic concepts related to DFS, be sure to view the introductory Flash demonstration named Dfs.swf, which you can access by visiting http://www.microsoft.com/windowsserver2003/evaluation/demos/dfs.html. Although this demonstration was created for Windows Server 2003, the fundamental concepts about DFS have not changed.


DFS is made up of the following network elements:

  • Namespace The virtual view of shared folders in an organization. A namespace is made up of the remaining elements on this list.

  • Namespace server A namespace server hosts a namespace. A namespace server can be a standalone server, a domain member server, or a domain controller.

  • Namespace root The namespace root is the starting point of the namespace. A domain-based namespace can be hosted on multiple namespace servers to increase the availability of the namespace.

  • Folder A container in a namespace that redirects clients to a folder target.

  • Folder targets A location separate from a folder in which data and content is stored.

The elements that make up a DFS namespace are illustrated in Figure 1.

Figure 1. DFS namespace elements

When you create a new namespace, you can create it as either a domain-based namespace or a standalone namespace. A domain-based namespace is published to Active Directory Domain Services (AD DS) and supports the file replication and built-in fault tolerance features. A standalone namespace stores its configuration information in the Registry of the namespace target that hosts it. Standalone namespaces do not integrate with AD DS and are stored on a single namespace server. Standalone namespaces do not support file replication.

When you create a namespace in Windows Server 2008 mode, two enhancements are added. First, Windows Server 2008 domain-based namespaces support increased scalability (more than 5000 folders). In addition, Windows Server 2008 namespaces support access-based enumeration. (With access-based enumeration, users can see on a file server only the files and folders for which the users have proper permissions.)

To create a domain-based namespace in Windows Server 2008 mode, your servers and domain will need to meet the following requirements:

  • The domain functional level must be Windows Server 2008.

  • All servers hosting the namespace must run Windows Server 2008.

DFS Component Technologies

In Windows Server 2008, DFS is based on two underlying technologies: DFS Namespaces and DFS Replication.

  • DFS Namespaces allow administrators to group shared folders located on different servers and present them to users as a virtual tree of folders known as a namespace. A namespace provides numerous benefits, including increased availability of data, load sharing, and simplified data migration.

  • DFS Replication is a multimaster replication engine that supports replication scheduling and bandwidth throttling. DFS Replication uses a compression protocol called Remote Differential Compression (RDC), which can be used to efficiently update files over a limited-bandwidth network. RDC detects insertions, removals, and rearrangements of data in files, thereby enabling DFS Replication to replicate only the changes when files are updated. Another important feature of DFS Replication is that in choosing replication paths, it leverages the Active Directory site links configured in Active Directory Sites and Services.

Figure 2 illustrates how DFS Namespaces and DFS Replication work together. In step 1, client computers contact a namespace server and receive a referral. In step 2, client computers access the first server provided by their referrals. The actual targets on the hosting servers are replicated with each other to allow local referrals.

Figure 2. DFS component technologies

More Info: DFS Replication

To see a demonstration of a DFS deployment across a branch office and witness some of the features of DFS Replication, it is highly recommended that you watch the 14-minute presentation by Drew McDaniel available at http://www.microsoft.com/winme/0512/25905/Branch_Server_demo_mbr.asx.


More Info: DFS

For a full introduction to DFS, read “Overview of the Distributed File System Solution in Windows Server 2003 R2,” available at http://go.microsoft.com/fwlink/?LinkId=55315. Although this paper deals with the version of the distributed file system in Windows Server 2003 R2, the underlying concepts are the same as those in Windows Server 2008.


DFS Namespaces Advanced Settings and Features

You can customize or enable the following settings and features in DFS Namespaces as necessary to design a DFS Namespaces solution for your organization.

Referral Ordering

A referral is an ordered list of targets, transparent to the user, that a client receives from a domain controller or namespace server when the user accesses the namespace root or a folder with targets in the namespace. The client caches the referral for a configurable period of time.

Targets in the client’s Active Directory site are listed first in a referral. (Targets given the target priority “first among all targets” will be listed before targets in the client’s site.) The order in which targets outside of the client’s site appear in a referral is determined by one of the following referral ordering methods:

  • Lowest cost

  • Random order

  • Exclude targets outside of the client’s site

You can set referral ordering on the namespace root, and the ordering method applies to all folders with targets in the namespace. You can also override the namespace root’s ordering method for individual folders with targets.

Failover and Failback

Client failover in DFS Namespaces is the process in which clients attempt to access another target server in a referral after one of the servers fails or is removed from the namespace. Client failback is an optional feature that enables a client to fail back to a preferred, local server after it is restored.

Failback occurs only when a client has failed over to a more expensive server (in terms of site link cost) than the server that is restored. If the restored server has the same cost as the server that the client is currently connected to, failback does not occur to the restored server. For example, if there are two servers (Server 1 and Server 2) in the client’s site and Server 1 fails while the client is connected to it, the client will fail over to Server 2. However, the client will not fail back to Server 1 when it is restored because both servers are located in the same site and therefore are associated with the same site link cost.

Note: Site link costs

You can view site link costs by using the Active Directory Sites and Services snap-in.


Target Priority

You can assign a priority to individual targets for a given namespace root or folder. This priority determines how the target is ordered in a referral. The options are:

  • First among all targets

  • Last among all targets

  • First among targets of equal cost

  • Last among targets of equal cost

It is important to note that setting target priority on a target will result in that target always being present in a referral, even in cases where you set the Exclude Targets Outside Of The Client’s Site option on the folder associated with the target.

Redundant Domain-Based Namespace Servers

Multiple namespace servers can host a domain-based namespace to increase the availability of the namespace. Putting a namespace server in remote or branch offices also allows clients to contact a namespace server and receive referrals without having to cross expensive wide area network (WAN) connections.

Namespace Scalability Mode

To maintain a consistent domain-based namespace across namespace servers, it is necessary for namespace servers to periodically poll AD DS to obtain the most current namespace data. If your organization will use more than 16 namespace servers to host a single namespace, it is recommended that you enable namespace scalability mode. When this mode is enabled, namespace servers running Windows Server 2003 and Windows Server 2008 do not send change notification messages to other namespaces servers when the namespace changes nor do they poll the PDC emulator every hour. Instead, they poll their closest domain controller every hour to discover updates to the namespace. (Regardless of whether namespace scalability mode is enabled, changes to the namespace are always made on the PDC emulator.)

Note: Root scalability mode

Namespace scalability mode was known as root scalability mode in Windows Server 2003.


DFS Replication Advanced Settings and Features

You can customize or enable the following settings and features in DFS Replication as necessary to design a DFS Replication solution for your organization.

RDC

RDC, which is the basis for DFS replication, is a protocol that can be used to efficiently update files 64 KB or larger over a limited-bandwidth network. RDC detects insertions, removals, rearrangements of data in files regardless of file type, enabling DFS Replication to replicate only the changes when files are updated. To compute the changes to replicate, RDC typically works on an older version of the file with the same name that exists at the appropriate location in the replicated folder tree on the receiving member.

In earlier versions of Windows Server the protocol used to replicate files among folders in a DFS namespace was File Replication Service (FRS). Unlike RDC, FRS copied only entire files, not portions of files. As a result, DFS in earlier versions of Windows is much more bandwidth-intensive than in Windows Server 2008 networks. This change in technology in Windows Server 2008 provides a huge improvement in DFS replication performance, especially across WAN links. Therefore, when planning for DFS, you should plan to upgrade your DFS servers if DFS replication will occur across WAN links.

Note: RDC and small files

RDC is not used on files smaller than 64 KB; in this case the file is compressed before it is replicated. You can also disable RDC on connections that are in a LAN where network bandwidth is not contended.


Cross-File RDC

An additional function of RDC, known as cross-file RDC, can be used to further reduce bandwidth usage. Cross-file RDC is useful when a file exists on the sending member and not the receiving member but similar files exist on the receiving member. Instead of replicating the entire file, DFS Replication can use portions of files that are similar to the replicating file to minimize the amount of data transferred over the WAN. Cross-file RDC can use multiple files as candidate files for RDC seed data.

Replication Schedule and Bandwidth Throttling

DFS Replication supports replication scheduling and bandwidth throttling in 15-minute increments during a seven-day period. When specifying a replication window, you choose the replication start and stop times as well as the bandwidth to use during that window. The settings for bandwidth usage range from 16 kilobits per second (Kbps) to 256 megabits per second (Mbps) as well as full (unlimited) bandwidth. You can configure a default schedule and bandwidth that applies to all connections between members and optionally create a custom schedule and bandwidth for individual connections.

Because members of a replication group are often located in different time zones, it is important to consider the time zones of the sending and receiving members when you set the schedule. The receiving member initiates replication by interpreting the schedule either in Coordinated Universal Time (UTC) or in the receiving member’s local time, depending on which setting you choose. You can choose this setting for the replication group schedule and for custom schedules on individual connections.

Replication Filters

You can configure file and subfolder filters to prevent files and subfolders from replicating. Both types of filters are set on a per-replicated folder basis. You exclude subfolders by specifying their name or by using the asterisk (*) wildcard character. You exclude files by specifying their name or by using the asterisk (*) wildcard character to specify file names and extensions.

Staging Folder

DFS Replication uses staging folders to act as caches for new and changed files to be replicated from sending members to receiving members. Each replicated folder uses its own staging folder, and each staging folder has a configurable quota. The quota, which governs when files are purged based on high and low watermarks, must be carefully set based on each replicated folder’s replication activity and the disk space available on the server.

Conflict And Deleted Folder

DFS Replication uses a last writer wins method for determining which version of a file to keep when a file is modified on two or more members and each member has not seen the other’s version. The losing file is stored in the Conflict And Deleted folder on the member that resolves the conflict. The Conflict And Deleted folder can also be used to store files that are deleted from replicated folders. Each Conflict And Deleted folder has a quota that governs when files are purged for cleanup purposes.

Disabled Memberships

A membership defines the relationship between each replicated folder/member pair. Each membership has a status, either enabled or disabled. If you do not want a replicated folder to be replicated to certain members, you can disable the memberships for those members. Doing so allows you to replicate folders to only a subset of replication group members.


Overview of the DFS Design Process

If you decide to implement DFS, you can use the following general outline to plan your DFS design:

  1. Identify data to replicate.

  2. Make initial namespace decisions.

  3. Design the replication topology.

  4. Plan for high availability and business continuity.

  5. Plan for delegation.

  6. Design the namespace hierarchy and functionality.

  7. Design replication schedules and bandwidth throttling.

  8. Review performance and optimization guidelines.

  9. Plan for DFS Replication deployment.

More Info: Designing DFS

For a detailed description of DFS planning and design, visit http://technet.microsoft.com and search for an article entitled “Designing Distributed File Systems.”


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