Topic Last Modified: 2005-11-18

The Microsoft® Exchange Server Analyzer Tool reads the following registry entry to determine if the Replication Long Backfill Request Delay key has been modified from the default value:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS \<servername> \Public- <GUID of Public Store> \Replication Long Backfill Request Delay

If the Exchange Server Analyzer finds Replication Long Backfill Request Delay to be present, a non-default configuration message is displayed.

By default, Replication Long Backfill Request Delay does not exist. Backfilling occurs when a public folder store determines that it has not received all of 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 below 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:

If one server does not have all of the needed changes, Exchange Server 2003 selects the next server in the sorted list and sends a backfill request to that server as well. This process is repeated until all of the changes have been requested. This feature is only available in Exchange Server 2003 and later; previous 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.

The Replication Long Backfill Request Delay value represents the length of time that a store delays before sending a backfill request for data that is not available in the same Exchange site. The value, in seconds, must be between 900 and 2592000 seconds (30 days); the default value is 43200 seconds (12 hours).

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 Replication Long Backfill Request Delay 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 Replication Long Backfill Request Delay.

    The value for this registry entry is in seconds. To return to the default value, you should delete Replication Long Backfill Request Delay 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 (