Scalability of Combined CAS and Hub Roles in Exchange 2007

From the White Paper: Determining the Scalability of Combined Client Access and Hub Transport Server Roles in Exchange 2007 over at Technet. http://technet.microsoft.com/en-us/library/dd901772.aspx

Conclusion

We found that the Client Access server and Hub Transport server roles can be combined and supported in most environments. Based on the test results, we recommend the following hardware configuration for servers that are running both the Client Access and Hub Transport roles:

8 processor cores
12 GB of RAM
Pagefile size, RAM + 10MB (for systems with 8 GB of RAM or less, set pagefile to RAM * 1,5)

The tests showed that a server with 8 processor cores and 12 GB of RAM provides an optimal balance of server costs and scalability. This configuration allows a combined-role server to handle significantly more load than a 4-core server if the number of clients increases and if the clients access processor resources heavily. Clients such as Outlook Web Access and Exchange ActiveSync consume more of the processor resources of a Client Access server than do clients such as Outlook. Increasing processor resources and memory on a Hub Transport server allowed the server to handle an increased average message size. There was a direct correlation between an increase in average message size and greater processor use. Having more available memory also allowed the server to better handle transient events, such as transport queue growth.

Key considerations for supporting the two roles on a single Exchange server are:

All deployments should take advantage of the transport database maximum cache size recommendations. This helps avoid back pressure events. For more information, see the Exchange Team Blog article New maximum database cache size guidance for Exchange 2007 Hub Transport Server role.
Consider carefully both RAM and processor requirements when you’re planning server hardware. Be sure you understand the following areas of resource constraint:
- The Hub Transport role is primarily processor and disk intensive.
- The Client Access server role is primarily memory intensive.
- The combined Client Access and Hub Transport server is primarily processor constrained.
A single 4-core, 8-GB RAM, combined-role server can service the same load as two 2-core, 4-GB RAM, single-role servers.
A single 8-core, 16-GB RAM, combined-role server can service the same load as two 4-core, 8-GB RAM, single-role servers.
Based on these tests, we don’t recommend deploying servers with 2 processor cores or 4 GB or less of RAM for use as combined-role servers.