Because the Exchange Server 2007 architecture differs so greatly from the Exchange Server 2003 architecture, It's important to familiarize yourself with the changes prior to deploying Exchange Server 2007.
The transition to 64-bit hardware
The most widely known architectural change in Exchange Server 2007 is that it's designed to run only on 64-bit systems. Microsoft has created a 32-bit version of Exchange Server 2007, but it's completely unsupported and only intended for training purposes.
Microsoft made the decision to move Exchange Server to a 64-bit platform because of memory. Over the last few years, Exchange Server mailbox stores have exponentially increased in size. This is attributable to several factors, including email compliance and message retention requirements and lower-cost storage. When mailbox stores grow significantly, Exchange Server has a hard time allocating enough memory to manage the information store.
Previous versions of Exchange Server would only run on 32-bit operating systems, which can address a maximum of 4 GB of memory. Under normal circumstances, Windows Server uses 2 GB for kernel-mode operations, leaving a mere 2 GB of memory for Exchange Server. In some situations, it is possible to use the /3GB switch to allocate an additional gigabyte of memory to user mode processes, but even this is sometimes barely enough.
In contrast, a 64-bit operating system can address up to 16 exabytes (EB) of memory. Current server hardware does not support installing the full 16 EB of RAM, but this simply means that there is room for future growth.
Exchange Server storage groups and databases
Storage groups have been overhauled in Exchange Server 2007 too. Exchange Server 2003 Enterprise Edition allows you to create up to four storage groups, each of which can contain up to five individual stores, for a total of up to 20 stores. In Exchange Server 2007, Microsoft has increased this limit, allowing up to 50 storage groups and 50 stores per server.
You still have the option of placing up to five stores within a single Exchange Server storage group, but the limitation of 50 total stores per server still applies regardless of how many Exchange Server storage groups are being used.
With the ability to create up to 50 storage groups in Exchange Server 2007, you can have a dedicated storage group for each information store. All stores within an Exchange Server storage group share a common set of log files. By placing each store into its own Exchange Server storage group, each store can have its own dedicated log files. This improves efficiency for the stores and, in some situations, for disaster recovery.
Exchange Server administrative groups
In Exchange Server 2003, administrative groups offer the ability to delegate control over a subset of the Exchange Server organization to a particular administrator. While this is a handy feature, Exchange 2003 administrative groups aren't exactly flexible. For example, it's impossible to move a server between administrative groups. Once a server is a member of a particular administrative group, it's stuck there.
In Exchange Server 2007, Microsoft has done away with administrative groups and replaced them with a new model for implementing administrative permissions that allows you to implement administrative rights at the organization level, the server level, or just about anywhere in between.
Exchange Server routing groups
Routing groups have also been removed from Exchange Server 2007. In Exchange 2000 Server and Exchange Server 2003, routing groups serve as a mechanism for providing backward compatibility with Exchange 5.5, which uses sites as a basis for determining a server's location within the Exchange Server organization.
Exchange Server 2007 does not support co-existence with Exchange 5.5 , so it has no need for backward compatibility with Exchange 5.5 sites.
Since Active Directory already contains its own site structure (which is different from the Exchange 5.5 site structure), and since Exchange Server routing groups almost always mimic the Active Directory site structure, Microsoft decided to do away with routing groups. Exchange Server 2007 simply relies on the Active Directory site structure instead of routing groups.
About the author: Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Exchange Server, and has previously received Microsoft's MVP award for Windows Server and Internet Information Server (IIS). 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 Web site at http://www.brienposey.com.
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