Windows 7 / Networking

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Planning for DFS Replication

When an organization wants to replicate data stored on Windows Server 2008 R2 systems published in DFS namespaces, administrators must create the namespaces on servers that are members of an Active Directory domain. Replication can be configured between multiple targets on a DFS folder or on Windows Server 2008 or Windows Server 2008 R2 systems that do not participate in a DFS namespace. When multiple targets are defined for a folder, DFS can utilize the FRS or the DFSR service to create replication connection objects and automatically synchronize data between each target.

Initial Master

When replication is first configured using the DFS console and the New Replication Group Wizard, the administrator can choose which target server will be the initial master. The data contained on the initial master is replicated to the remaining targets. For targets on servers other than the initial master, existing data is moved to a hidden directory, and the current folder is filled with the data contained only in the initial master folder. After initial replication is complete, the administrator can restore data moved to the hidden folder back to the working directory, where it can trigger replication outbound to all the other replicas in the replica set, if replication is two-way and neither target is set to readonly. As a best practice, when adding additional targets to a replica set, try to start with empty folders.

The Staging Folder

The staging folder is the location where a DFS Replication member stores the data that will be replicated to other replication members within a replication group. In a fully synchronized replication group, the staging folder on all servers will be empty. Because replication data will travel through this folder, the drive hosting the staging folder must have sufficient free space to accommodate the maximum size of the staging folder and should be able to handle the additional disk load. By default, the staging folder is limited to 4GB. The default location for the staging folder will be located in the target share folder in a hidden directory named c:\Apps\DfsrPrivate\Staging, if the target is located in c:\Apps as an example. The staging folder and its 4GB limit is unique to each target on each server.

Determining the Replication Topology

Windows Server 2008 R2 DFS provides a number of built-in replication topologies to choose from when an administrator is configuring replication between DFS folder targets or replication group members; they're described next. As a general guideline, it might be prudent to configure DFS Replication connections and a schedule to follow current Active Directory site replication topology connections or the existing network topology when the organization wants true multimaster replication.

Hub and Spoke

A hub-and-spoke topology is somewhat self-descriptive. A single target is designated as the replication hub server, and every other target (spoke target) replicates exclusively with it. The hub target has two replication connections with each spoke target: sending and receiving. A hub-and-spoke topology requires three or more servers, and when the hub target is unavailable, replication updates stop between all replication members. Windows Server 2008 R2 introduces the ability to specify more than one hub when creating a hub-andspoke replication topology. In previous releases, this required creating a custom topology.

Full Mesh

Using a full mesh topology, each target has a connection to every other target in the replication group. This enables replication to continue among available replication members when any member becomes unavailable. Because each member has a connection to every other member, replication can continue with as few as two replication members. Using this topology with read/write replication sets can lead to data conflicts if data is being changed in multiple sites so this topology should be used with caution.

No Topology and Custom Topology

During the creation of a replication group, one of the topology options is the No Topology option. Selecting this option enables an administrator to create a custom replication topology after the replication group is created. A custom topology allows an administrator to define specific replication connections for each target. This option can be useful if an organization wants to define one-way replication for centralized backup or to optimize read-only replicated folders. Also, this can be most useful when creating a topology for a network that is connected using different speed WAN links or each connection needs to have a specific schedule and bandwidth setting.

Replication Schedule and Bandwidth Throttling

Windows Server 2003 R2, Windows Server 2008, and Windows Server 2008 R2 DFS Replication support scheduling replication, as well as restricting the amount of bandwidth a replication connection can utilize. In the original version of DFS that came with Windows 2000 and the initial release of Windows 2003, administrators were limited in their replication scheduling options and forced to limit replication to after hours for large data sets as opposed to trickling data replication all day long using only a portion of the wide area network (WAN) link between sites. For large data sets that will initially replicate across the WAN, the initial replication connections can be configured to run limited bandwidth during business hours and full bandwidth after hours until replication has completed and restrictions can be removed if desired.