Placing Multiple Targets
Updated: March 28, 2003
Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2
When you evaluate where to place multiple targets, you should plan to place at least one target in the same Active Directory site where users access the data. Doing so enables clients to use fast, inexpensive bandwidth to access the target. Use multiple same-site targets to ensure namespace availability in each site and to avoid the need to use expensive bandwidth if one of the same-site targets fails or is taken offline. Using multiple same-site targets also provides load sharing among the targets in the site.
After you determine where to place multiple targets, you also need to consider where clients will be redirected if the primary target is unavailable. DFS supports three methods of target selection, which are described in the following sections. These methods apply to both stand-alone and domain-based DFS namespaces. After you determine the method of target selection for each link or root, document your decisions. For an Excel spreadsheet to assist you in documenting the target selection methods, see "DFS Configuration Worksheet" (Sdcfsv_1.xls) on the Windows Server 2003 Deployment Kit companion CD (or see "DFS Configuration Worksheet" on the Web at http://www.microsoft.com/reskit).
If you have a mix of root servers running Windows 2000 Server and Windows Server 2003, target selection is random if a root server running Windows 2000 Server provides a referral for a link target created in Windows Server 2003, regardless of which target method you configure. For more information about target selection, see "Designing a DFS Namespace" earlier in this chapter.
Default target selection
If the last (or only) target in an Active Directory site fails or is taken offline, DFS directs clients to another target in the same site, if a target is available. Clients are directed to a random target if no same-site targets are available. DFS does not consider bandwidth cost, connection speed, or the target server’s processing load when choosing the random target.
Restricted same-site target selection
By using Dfsutil.exe /InSite parameter, you can limit client access to only those targets that are in the same site as the client. You enable this feature on a DFS root or on individual links in the namespace. If you enable this feature on the root, referrals for any link in the namespace return only targets that are in the same site as the client. If this functionality is disabled on the root, the individual settings on each link are used. When using this feature, plan to have at least one target (or two targets, for fault tolerance) in every site, and plan to monitor servers to make sure they are online and accessible. If no same-site targets exist, clients in that site are denied access to the data in the namespace.
The /InSite parameter takes effect after you stop and restart the Distributed File System service on each root server or when the service reads the DFS metadata, which happens every hour by default.
Least expensive target selection
If you create a stand-alone or domain-based DFS root on a server running Windows Server 2003, and the domain controller acting as the Intersite Topology Generator (ISTG) is also running Windows Server 2003, you can use the /SiteCosting parameter in Dfsutil.exe to enable DFS to choose an alternate target based on connection cost if no same-site targets are available. Windows Server 2003 uses the site and costing information in Active Directory to determine whether sites are linked by inexpensive, high-speed links or by expensive WAN links.
Site costing is not available in the following situations:
When a stand-alone DFS namespace is hosted on a server that is not part of any domain.
When the closest domain controller acting as the ISTG is running Windows 2000 Server.
(This situation occurs when there are only domain controllers running Windows 2000 Server in the site of the DFS root server, or when there are no domain controllers in the site of the DFS root server and the closest site with at least one domain controller has only Windows 2000 Server domain controllers in that site. The closest site is defined in Active Directory.)
If you plan to enable site costing, review the following:
You can enable site costing on a per-namespace basis.
When the domain controller acting as the ISTG is running Windows 2000 Server, DFS uses the default target selection, described earlier in this section. If domain controllers running Windows 2000 Server and Windows Server 2003 exist in a site, the ISTG role is automatically given to the domain controller running Windows Server 2003.
For more information about defining sites in Active Directory, see "Designing the Site Topology" in Designing and Deploying Directory and Security Services of this kit.