Potential coexistence troubles during Exchange 2010 migrations

Coexistence issues are a natural part of Exchange 2010 migrations, so keep an eye on client access servers and mailbox databases during the entire upgrade.

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If you’re migrating from Exchange Server 2007 to Exchange 2010, several coexistence issues are bound to happen. In this tip, Exchange expert Brien Posey explains why client access server coexistence may not be that terrible, though the disappearance of single instance storage should be cause for concern.


A big Exchange 2007 to Exchange Server 2010 migration complaint has to do with Microsoft’s guidelines for client access server (CAS) coexistence. An Exchange team blog post on dealing with CAS coexistence discusses issues like why you need to assign new URLs to the Exchange 2007 CAS and why you need to acquire new SSL certificates.

Most issues the Microsoft team addresses involve Exchange organizations that use multiple Active Directory sites. Environments with Internet-facing CAS servers located behind an HTTP proxy such as an ISA Server or ForeFront Threat Management Gateway can also experience problems.

Organizations with a single Active Directory site that only use the CAS server to access OWA have it easy since OWA 2010 is incompatible with Exchange 2007 mailboxes. If a user attempts to log into OWA via an Exchange 2010 CAS, he will automatically be redirected to an Exchange 2007 client access server. If you only use OWA internally, you don’t have to perform any special configurations; however, users will need to learn a new OWA URL.

If OWA is accessible from outside the organization, you may have to point the OWA URL to your Exchange 2010 CAS server. You’ll also need to make sure that the new CAS server’s SSL certificate matches the URL. Lastly, verify that the Exchange 2007 CAS server certificate is still valid after redirecting the external URL.

Effects of removing single instance storage in Exchange 2010

As Exchange server mailboxes increase in size and number, I/O requirements grow exponentially. Since Exchange Server 2010 was designed to be scalable enough to host millions of mailboxes, it was important to drive down I/O requirements.

Unfortunately, database performance improvements come at a price. By simplifying the database, Microsoft eliminated single instance storage (SIS). Previously, if a user sent a 1 MB message to 100 people, it would only consume 1 MB of within the database because only one copy of the message would have been stored. However, if the user sent the same message to 100 Exchange Server 2010 recipients, the message would consume 100 MB of space because Exchange stores 100 copies of the message.

Typically, 100 MB isn’t a lot by today’s standards, but a single message consuming 100 MB is a big deal. Companies migrating from Exchange 2007 will find that mailboxes grow after they’ve have been moved; and mailbox databases grow much more quickly than they did in Exchange 2007.

This design change may have other consequences beyond bloated databases. You may find that you need to increase mailbox quota sizes to accommodate the new database format. You might also need to rethink your backup strategies.

About the author: Brien M. Posey, MCSE, is a seven-time recipient of Microsoft's Most Valuable Professional (MVP) award for his work with Exchange Server, Windows Server, Internet Information Services (IIS), and File Systems and Storage. Brien has served as CIO for a nationwide chain of hospitals and was once responsible for the Department of Information Management at Fort Knox. As a freelance technical writer, Brien has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal website at www.brienposey.com.

This was first published in September 2010

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