Topic Last Modified: 2008-01-16

The Microsoft Exchange Server Analyzer Tool reads the following registry entry to determine the value of the ClusterLogSize parameter: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Session Manager\Environment\

If the Exchange Analyzer finds that the ClusterLogSize environment variable is set to a value lower than the recommended value of 32 megabytes (MB), the Exchange Analyzer displays a best practices message.

On a busy cluster, the cluster.log file wraps around so quickly that entries may be lost when you troubleshoot failover situations if the following conditions are true:

Therefore, when troubleshooting requires use of the cluster.log file, it is a recommended best practice to increase the size to at least 32 megabytes (MB).

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 modify the default size of the cluster.log file
  1. Open a registry editor, such as Regedit.exe or Regedt32.exe.

  2. Navigate to: HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Environment.

  3. Right-click ClusterLogSize, and then click Modify.

  4. Modify the ClusterLogSize Value data to the new log file size. For example, modify the ClusterLogSize value to 32 and then press ENTER.

  5. Quit Registry Editor.

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

For more information about cluster logging in Microsoft Cluster Server, see the Microsoft Knowledge Base article 168801, "How to Turn on Cluster Logging in Microsoft Cluster Server" (