Topic Last Modified: 2005-11-18

The Microsoft® Exchange Server Analyzer Tool reads the following registry entry to determine whether the threshold for logging an event regarding virtual memory fragmentation has been manually overridden:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeIS\ParametersSystem\VM Warning Level

If the Exchange Server Analyzer finds the VM Warning Level registry value present and configured, a non-default configuration message is displayed.

In 32-bit Microsoft Windows® systems, there are two kinds of memory: physical memory (the amount of actual memory installed in the system) and virtual memory address space (a virtualized 4 gigabyte memory space). Regardless of how much physical memory is installed on the system, the total size of the virtual address space is always the same: 4 GB. Although it is not the case for all 32-bit applications, for Exchange Server, virtual address space equals virtual memory; in other words, the terms are used interchangeably.

During the regular course of its operations, the Exchange information store will repeatedly allocate, use, and de-allocate virtual memory. When virtual memory is allocated, used, and de-allocated frequently, it can cause virtual memory fragmentation. Virtual memory fragmentation is analogous to file system fragmentation that can occur over time. After a while, allocated segments of virtual memory are no longer contiguous, and performance begins to suffer. For the file system, you can run an application that performs defragmentation of the file. Generally, you can do this while the system is online and being accessed. For virtual memory fragmentation, there is no defragmentation tool; instead, to correct the problem you must stop and start the Microsoft Exchange Information Stores service or restart the system.

By default, when an Exchange Server computer has less than 32 MB of free contiguous virtual address space, the following warning message is logged to the application event log:










The virtual memory necessary to run your Exchange server is fragmented in such a way that performance may be affected. It is highly recommended that you restart all Exchange services to correct this issue.

The VM Warning Level registry value represents a configuration opportunity that lets an administrator override the default 32 MB level warning and set an alternate value.

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 revert to the default configuration

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

  2. Navigate to:


  3. In the right-hand pane, delete the value for VM Warning Level.

  4. Close the registry editor and restart the Microsoft Exchange Information Store service for the change to take effect.

For more information about troubleshooting virtual memory fragmentation in Exchange Server 2003 and Exchange 2000 Server, see the Microsoft Knowledge Base article 325044, "HOW TO: Troubleshoot Virtual Memory Fragmentation in Exchange 2003 and Exchange 2000" (