XADM: Bad Schema Index After Exchange 4.0 to 5.5 Upgrade

Users experience Exchange service outages from roughly 20 seconds to 4 minutes long sporadically throughout the day. Threads in Dsamain.exe may spike to 100 percent.

Under Microsoft Exchange Server 4.0, the schema index for the directory service defaults to 0. This amounts to a flat scan for every directory service request. The algorithm has changed significantly from 4.0 to 5.5 for the database retrieval mechanism, including improved caching. Under version 5.5, a flat scan has a significant performance cost and hence the index defaults to 1 by design. However, when you install version 5.5 in a version 4.0 environment or upgrade 4.0 to 5.5, this value remains 0 and causes the server outage described above.

Microsoft has confirmed this to be a problem in Microsoft Exchange Server version 5.5. This problem has been corrected in the latest U.S. Service Pack for Microsoft Exchange Server version 5.5. For information about obtaining the Service Pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

To work around this problem, set the directory service schema index to 1 for Exchange Server 5.5 computers in a mixed 4.0/5.5 environment. To do this, perform the following steps:

WARNING: Using the raw mode of the Exchange Administrator program (admin /r) incorrectly can cause serious problems that may require you to reinstall Microsoft Windows NT Server and/or Microsoft Exchange Server. Microsoft cannot guarantee that problems resulting from the incorrect use of raw mode can be solved. Use raw mode at your own risk.

1. Start the Exchange Administrator program in raw mode by typing the following at a command prompt:
   admin /r
2. On the View menu, click Raw Directory.
3. Select the Schema object in the Exchange Administrator hierarchy. This is located under the site name.
4. Open the Properties window for the Directory Name schema object.
5. In the Object Attribute list, click Search-Flags.
6. Change the Edit Value to 1. Then select Set.
7. Restart the directory service.

NOTE: This change must be made on one server within each site experiencing the problem. This change will be replicated to all the servers within the site.