Scalability is the ability for a system to grow to meet increasing performance demands. When applied to clustering, scalability is the ability to incrementally add systems to an existing cluster when the overall load of the cluster exceeds the cluster's capabilities—either by scaling up or scaling out. Scaling up involves increasing system resources (such as processors, memory, disks, and network adapters) to your existing hardware or replacing existing hardware with greater system resources (for example, faster CPU and network adapter, more memory, and more storage). Scaling out involves adding servers to meet demand. For more information about scalability strategies, see the Exchange Server 2003 High Availability Guide.

This topic discusses how Exchange Server 2003 scales under different configurations and user loads.

Implementing a front-end and back-end server environment presents different challenges that affect your overall performance.

Front-end servers, such as those that serve Microsoft Outlook® Web Access, Outlook Mobile Access, Exchange ActiveSync®, RPC over HTTP, authentication, IP address checking, Secure Sockets Layer (SSL) protocol, and encryption schemes, have security features that require significant processing. For these servers, you are likely to see an increase in processor activity, both in privileged and user mode, and an increase in the rate of context switches and interrupts. If the processors in the server cannot handle this increased load, queues are likely to develop.

Factors that affect front-end servers include:

• The protocols being used.
  
• The number of processors installed.
  
• The available memory.
  
• The network traffic.
  
• The authentication methods.
  
• The use of SSL to encrypt network traffic.
  

Because front-end servers forward all requests to the back-end servers, back-end servers have the same processor and processing issues that front-end servers have. Back-end servers may also experience storage issues because of the read and write activity when retrieving and storing data. With public folders, replication traffic between public folders (if there is more than one public folder in the topology) can affect all the servers involved.

Factors that affect back-end servers include:

• The protocols being used.
  
• The number of processors installed.
  
• The available memory
  
• The type of storage used.
  
• The storage available.
  
• The replication of public folder information.
  

For more information about how these factors affect front-end and back-end servers, see "Baseline Data" later in this topic.

Microsoft Windows® 2000 Server and Windows Server™ 2003 support two types of licensing models: per seat and per server. The Windows 2000 License Logging Service maintains a list (on disk and in memory) of all users who authenticate against a server using the per-seat licensing model. The per-server model does not keep a list of users. The list of authenticated users in the per-seat configuration does not consume too much memory on servers with fewer than 50,000 users. However, the memory footprint of the License Logging Service can grow too large when it runs with a per-seat licensing model in a front-end and back-end topology, with hundreds of thousands of users.

In the per-seat scenario, with the front-end servers load balancing the client requests, the front-end server's License Logging Service builds a list of all users in the site (including all users on all back-end servers). Depending on the size of the site, the server can consume hundreds of megabytes (MB) of memory in the License Logging Service on the front-end server. Therefore, it is recommended that the per-server licensing model be used in large front-end and back-end topologies. For more information about the License Logging Service, see the Windows 2000 Resource Kits Web site (http://go.microsoft.com/fwlink/?LinkId=6545).

Because of the wide variety of Exchange configurations and user profiles, it is difficult to accurately determine the number of users supported by a server. You must consider the different types of clients, how active the users are, the capacity of the storage subsystem, and how the Exchange server is configured to use the disk resources. For more information about how to calculate your server sizing requirements, see Calculate Your Server Size.

This section describes how Exchange Server 2003 performs under different configurations and user loads. This information can help you establish a baseline when determining what minimum hardware you require. The following scenarios include:

• Mailbox Server (using MAPI)
  
• Outlook Web Access
  
• Post Office Protocol version 3 (POP3)
  
• Internet Message Access Protocol version 4rev1 (IMAP4)
  
• Simple Mail Transfer Protocol (SMTP)
  

Each scenario analyzes the following areas:

• Processor
  
• Memory
  
• Disk usage
  
• Network usage