Preferred backfill source enabled

[This topic is intended to address a specific issue called out by the Exchange Server Analyzer Tool. You should apply it only to systems that have had the Exchange Server Analyzer Tool run against them and are experiencing that specific issue. The Exchange Server Analyzer Tool, available as a free download, remotely collects configuration data from each server in the topology and automatically analyzes the data. The resulting report details important configuration issues, potential problems, and nondefault product settings. By following these recommendations, you can achieve better performance, scalability, reliability, and uptime. For more information about the tool or to download the latest versions, see "Microsoft Exchange Analyzers" at]  

Topic Last Modified: 2005-11-18

The Microsoft® Exchange Server Analyzer Tool reads the following registry entries to determine whether the Preferred Backfill Source key has been modified from the default value:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\<servername>\Public-<GUID of Public Store>\Preferred Backfill Source

By default, Preferred Backfill Source does not exist. If the Exchange Server Analyzer finds Preferred Backfill Source to be present, a non-default configuration message is displayed.

Backfilling occurs when a public folder store determines that it has not received all the updates for a replicated folder (or for the hierarchy) and must retrieve the missing updates from another store.

To streamline the backfill process, Exchange Server stores information about missing updates in the backfill array.

The backfill array stores this information for a specified length of time (called the backfill time-out). If the missing updates arrive in subsequent replication messages during this time, they are removed from the backfill array. The table lists the default backfill time-out values, which depend on where the missing updates exist and whether they have been requested before.

Type of request Content exists on a store in the local site Content exists on a store in a remote site

Initial backfill

6 hours

12 hours

First backfill retry

12 hours

24 hours

Subsequent backfill retries

24 hours

48 hours

If the backfill time-out expires and the updates are still missing, Exchange Server 2003 creates one or more backfill requests and determines which servers to use as backfill sources.

To select a server (or servers) to use as a backfill source, Exchange Server 2003 first creates a list of all of the servers that have replicas of the folder, and then sorts the list according to the following sequence of criteria:

  • Sort according to server status. Servers that are down or unavailable drop to the end of the list.

  • Sort according to preferred backfill server. (if any; this feature is available only in Exchange Server 2003 and later). Exchange Server 2003 checks the public folder store object in the Active Directory® directory service for a preferred backfill server. This setting is seldom used. In most circumstances, the backfill process operates most efficiently if Exchange Server 2003 selects a backfill server automatically. Most deployments of Exchange Server 2003 do not need a preferred backfill server. Microsoft Product Support Services can provide a script that sets a preferred backfill server if your deployment requires it.

  • Sort according to transport cost (lowest to highest). Servers in the same site have priority over servers in remote sites. The transport cost of a server is calculated by the Exchange Server 2003 routing engine, and is normally used to calculate the most efficient way to deliver a message. Note that in versions of Exchange prior to Exchange Server 2003, transport cost has lower priority and is only considered after Exchange version and number of available changes.

  • Sort according to Exchange version (newest to oldest).

  • Sort according to the number of necessary changes that are available on the server (largest to smallest). Servers that do not have any of the missing changes are dropped from the list.

If one server does not have all the needed changes, Exchange Server 2003 selects the next server in the sorted list and sends a backfill request to that server also. This process is repeated until all the changes have been requested. This feature is only available in Exchange Server 2003 and later; versions of Exchange send backfill requests to one server at a time if no single server holds all of the missing updates for a specific folder. After each request, previous versions of Exchange wait for the retry time-out (from 24 through 48 hours) to elapse before sending the next request.

If the selected server does not respond to the backfill request, the store marks that server as down and repeats the selection process. Servers marked down drop to the end of the list.

Most of the time, your topology will operate efficiently if you allow Exchange Server to automatically select backfill servers. For troubleshooting purposes, Microsoft Product Support Services may recommend setting Preferred Backfill Source. However, you should only use it when necessary to resolve a specific problem.


This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore the registry if a problem occurs. For information about how to restore the registry, view the "Restore the Registry" Help topic in Regedit.exe or Regedt32.exe.

To reset the Preferred Backfill Source registry key

  1. Open a registry editor, such as Regedit.exe or Regedt32.exe.

  2. Navigate to: HKLM\System\CurrentControlSet\Services\MSExchangeIS\<servername>\Public-<GUID of Public Store>**

  3. Examine or edit the value for Preferred Backfill Source.

    The value for this registry entry is in kilobytes (KB). To return to the default value, you should delete Preferred Backfill Source and restart the Microsoft Exchange Information Store service.

Before you edit the registry, and for information about how to edit the registry, see the Microsoft Knowledge Base article 256986, "Description of the Microsoft Windows Registry" (

For more information about public folder replication, see "Controlling Exchange Server 2003 Public Folder Replication" in Working with the Exchange Server 2003 Store (