Topic Last Modified: 2005-11-18

The Microsoft® Exchange Server Analyzer Tool queries the Win32_PerfRawData_MSExchangeIS_MSExchangeIS Microsoft Windows® Management Instrumentation (WMI) class to determine the value for the VMLargestBlockSize key. The VMLargestBlockSize key represents the size (in bytes) of the largest available block of virtual memory.

Also, the Exchange Server Analyzer also queries the Active Directory® directory service to determine the count of the entries listed in the homeMDBBL attribute of each mailbox store. The count of this attribute represents the number of mailboxes on the mailbox store.

If the Exchange Server Analyzer finds the value for VMLargestBlockSize more than 50000000 but less than 150000000 on an Exchange Server computer with more than 20 mailboxes, a warning is displayed.

Among other things, Exchange Server uses virtual memory to store messages when you convert them between MIME and MAPI formats, and as a buffer when committing messages from the transaction log and committing to the store. Frequently these operations require large contiguous chunks of virtual memory to complete quickly and efficiently. If this warning has been displayed, the virtual memory on the Exchange computer is depleted to less than 150 MB of contiguous space. This is known as virtual memory fragmentation.

If the virtual memory depletes too much, you may start to see error messages in the event log (Event ID 9582), which are virtual memory errors. If virtual memory is low enough, you will see Event ID 12800, which means messages are not being delivered because there is insufficient memory to process messages.

To resolve this warning, follow these recommendations in the order in which they are presented.

Look for error, "Maximum ESE cache size raised," in the Exchange Server Analyzer output.

If the error, Maximum ESE cache size raised, is returned on the same Exchange computer where this virtual memory fragmentation warning is returned, follow the recommendations in the article, Maximum ESE Cache is set too high.

The coupling of these two messages likely indicates that the virtual memory fragmentation issues are a result of the ESE cache being manually increased

Restart Exchange Information Store service

In the description of this error message, the Exchange Server Analyzer specifies the number of days that the Microsoft Exchange Information Store service has been running. If the number of days specified is more than 30, restart the service.

To restart the Microsoft Exchange Information Store service

  1. Open the Services MMC snap-in.

  2. Right-click Microsoft Exchange Information Store service, and then click Restart.

Evaluate message size limits

If you have many users sending large messages (more than 30 MB), virtual memory may become fragmented sooner than it otherwise might have if many users did not send large messages. As mentioned, virtual memory is used as a buffer when the store process moves data from log files and commits it to the store. If the data being moved is frequently large, then big, contiguous chunks of virtual memory are consumed quickly.

If large data is regularly sent using e-mail, and that data is sent internally, consider setting up shared directories on file servers where users can post large pieces of data.

It is recommended setting the maximum message size limit to no more than 30 MB. However, if you need a higher message size limit for messages with large attachments to be sent through your Exchange organization, you may want to increase the maximum outgoing message size limit above 30 MB. In addition to virtual memory fragmentation, increasing the message size limit over 30 MB can cause your system to be more vulnerable to malicious attacks. It is not recommended to allow more than a 30 MB message size limit.

To set message size limits

  1. In Exchange System Manager, expand Global Settings, right-click Message Delivery, and then click Properties.

  2. In the Message Delivery Properties dialog box, on the Defaults tab, select the Sending message size limit option and enter the size limit you want.

Verify that antivirus software is up-to-date

Sometimes, outdated antivirus software that is being run in Exchange Virus Scanning API (VSAPI) mode will cause virtual memory fragmentation. If you are running antivirus software in VSAPI mode, contact your antivirus vendor and verify that you are running the latest version of their software and drivers.

Move mailboxes to another Exchange server

If you have worked through all the procedures, and the Exchange Server Analyzer is still displaying this warning, you must move mailboxes off of the affected server to reduce its load. There are too many mailboxes on this server.

To move mailboxes on Exchange 2000 Server and Exchange Server 2003

  1. In Active Directory Users and Computers, select the user or users whose mailboxes you want to move.

  2. Right-click the user list you selected in the previous step, and then click Exchange Tasks.

  3. In the Exchange Task Wizard, on the Available Tasks page, click Move Mailbox, and then click Next.

  4. Carefully read and follow the remaining steps in the wizard.

To move mailboxes on Exchange Server 2003

  1. In Exchange System Manager, expand Servers, expand the server from which you want to move mailboxes, expand the Storage Group from which you want to move mailboxes, expand the Mailbox Store that contains the mailboxes that you want to move, and then click Mailboxes.

  2. In the details pane, right-click the user or users whose mailboxes you want to move, and then click Exchange Tasks.

  3. In the Exchange Task Wizard, on the Available Tasks page, click Move Mailbox, and then click Next.

  4. Carefully read and follow the remaining steps in the wizard.

For more information about resolving virtual memory fragmentation issues, see the following Microsoft Knowledge Base articles:

For more details about setting message size limits, see the Knowledge Base article 322679, "Set Size Limits for Messages" (

For more information about moving mailboxes, see the following Knowledge Base articles: