Exchange Server experiences performance issues when a PDC emulator is used for DSAccess or ADAcess

By default, DSAccess or ADAccess chooses the primary domain controller (PDC) emulator operations master role computer to handle requests in Microsoft Exchange. This action may result in poor performance if other non-Exchange programs are making heavy use of the PDC emulator.�

To resolve this issue, use one of the following methods, as appropriate for your situation.

Method 1: Microsoft Exchange 2000 Server and later versions

To resolve this problem in Exchange 2000 Server and later versions, add the MinUserDC registry value to exclude the PDC emulator from the server list that Exchange can use.

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
To add the MinUserDC registry value, follow these steps:

1. Start Registry Editor.
2. Locate and then click the following subkey in the registry:
   HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeDSAccess\Profiles\Default
   Note In Exchange Server 2007 and in Exchange Server 2010, locate the MSExchangeADAccess subkey instead of the MSExchangeDSAccess subkey.
3. On the Edit menu, click Add Value, and then add the following registry value:
   Value name: MinUserDC
   Data type: REG_DWORD
   Value data: As required
   
   Note The MinUserDC entry determines how many total user domain controllers must be available for PDC emulator exclusion to turn on. The value data that is configured for the MinUserDC registry entry is the maximum number of domain controllers to contact before the PDC emulator is contacted. For example, when you set MinUserDC to 4, this configures DSAccess to exclude the PDC emulator only when a total of four domain controllers are available. When this condition is met, the PDC emulator is excluded from use, and DSAccess communicates only with the remaining three domain controllers.
4. Exit Registry Editor.

Notes

• You have to apply the registry change that this article describes regardless of the service pack that is installed.
• In Exchange Server Enterprise Edition, the Profiles subkey and the Default subkey are not available. You must create these subkeys in Exchange Server Enterprise Edition.
• When you determine a value for MinUserDC, consider the equation n - 1, where n is equal to the total number of domain controllers in the site. This number includes the PDC emulator. Subtract 1 from this number, and the sum should be the value that you enter for MinUserDC.

Method 2: Exchange Server 2007 and Exchange Server 2010

In Exchange Server 2007 and in Exchange Server 2010, you can use a cmdlet to configure the ADAccess component to exclude a particular domain controller or a list of domain controllers from use. To do this, use the Set-ExchangeServer command together with the StaticExcludedDomainControllers option.

The following example shows how to use the Set-ExchangeServer command to exclude one or more domain controllers from use. Additionally, this example shows how to verify the status of the Exchange environment after you run the Set-ExchangeServer command.

In this example, you have the following servers: To use the Set-ExchangeServer command to exclude the three domain controllers that are listed in this table from use for the DSAccess component, follow these steps:

1. Start the Exchange Management Shell. To do this, click Start, point to All Programs, point to Microsoft Exchange Server 2007, and then click Exchange Management Shell.
2. At the command prompt, type the following command, and then press Enter:
   Set-ExchangeServer -identity E2K7-1.contoso.com -StaticExcludedDomainControllers:dc-1.contoso.com,dc-2.contoso.com,dc-3.contoso.com
   This command excludes DC-1, DC-2, and DC-2 from use by the server that is named E2K7-1.
   
   Note In this command, specify the fully qualified domain names of the individual domain controllers by using a comma-separated list that does not contain spaces between each entry.
3. To verify the list of excluded domain controllers, type the following command, and then press Enter:
   Get-ExchangeServer -identity E2K7-1.contoso.com -status | fl Name, StaticExcludedDomainControllers

Note If you want to remove the changes that you have made and revert to the default behavior of Exchange, type the following command at the Exchange Management Shell prompt, and then press Enter:

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

In Exchange Server 2007 and in Exchange Server 2010, you can use the Set-ExchangeServer cmdlet to configure the ADAccess component to exclude a particular domain controller or a list of domain controllers from use. You do not have to edit the registry when you use the Set-ExchangeServer cmdlet. However, you should use caution when you use the Set-ExchangeServer cmdlet to exclude a domain controller from use. For example, if your domain has Domain_Controller_A and Domain_Controller_B, and you use the Set-ExchangeServer cmdlet to exclude Domain_Controller_A from use, Exchange Server stops working if Domain_Controller_B is not available.

You can also add the MinUserDC registry value to exclude the PDC emulator from use by the ADAccess component. When you use the MinUserDC registry value, you can set a minimum level of domain controller resources before the server that has the PDC Emulator role is enabled to handle Exchange requests. The MinUserDC registry value has the following advantages over using the Set-ExchangeServer cmdlet:

• When you use the MinUserDC registry value, the domain controller that has the PDC emulator role is still available for use if all other domain controllers fail. If you use the Set-ExchangeServer cmdlet, the domain controller that has the PDC emulator role is permanently excluded.
• When you use the MinUserDC registry value and then move the PDC Emulator role to a different domain controller, the domain controller that now hosts the PDC Emulator role is automatically excluded from use by the ADAccess component, and the domain controller that previously hosted the PDC Emulator role is available to handle Exchange requests.
  
  If you use the Set-ExchangeServer cmdlet, and the PDC Emulator role is moved to another domain controller, you must run the Set-ExchangeServer cmdlet again to update the StaticExcludedDomainControllers list. If you do not run the Set-ExchangeServer cmdlet again, the domain controller that hosts the PDC Emulator role after the move will be available to handle Exchange requests. Additionally, the domain controller that previously hosted the PDC Emulator role will still be excluded.

For more information, click the following article number to view the article in the Microsoft Knowledge Base:

For more information about the Set-ExchangeServer command, visit the following Microsoft website:

http://technet.microsoft.com/en-us/library/bb123716.aspx