Extensive testing on dual-core processors has shown that Exchange Server benefits significantly when it uses multi-core processor technology. The performance benefit for Exchange Server from dual-core technology depends upon the specific processor used. The findings from Exchange Server 2003 dual-core testing are summarized in Microsoft Knowledge Base article 827281, CPU and memory scalability for Exchange Server 2003 and for Exchange 2000 Server. In addition, the performance benefit of specific dual-core implementations can be seen by comparing the MAPI Messaging Benchmark 3 (MMB3) results of a four-processor, single-core-based server to a two-processor, dual-core-based server. These results are published in Performance Benchmarks for Computers Running Exchange Server 2003. These benchmarks are currently being updated for Exchange 2007, and Exchange 2007 benchmarks will be published when they are available.

Today, multi-core processors are an attractive option for Exchange 2007 based on price and performance. We recommend that you ask your hardware vendor about dual-core benefits for Exchange Server for your specific hardware platform.

The Edge Transport server role is extremely efficient in design and thus requires moderate processing power. In addition, organizations requiring fault tolerance will deploy multiple Edge Transport servers to provide redundancy. The recommended configuration of 2 x processor cores assumes a fault-tolerant deployment. Large organizations, with a substantial volume of inbound and outbound messages, will be able to use servers with 4 x processor cores to reduce the aggregate Edge Transport server count. Processor utilization is based on several factors such as: message rate, average message size, number of enabled transport agents, antivirus configuration, and third-party applications.

The recommended configuration for the Hub Transport server role is 4 x processor cores in organizations where Hub Transport servers are deployed with several Mailbox servers and thousands of mailboxes. 8 x processor core servers can be efficiently used when the Hub Transport server is configured to use antivirus and anti-spam. Either 1 x or 2 x processor core configurations can be considered for organizations where there are not enough mailboxes or insufficient message traffic to warrant using a 4 x processor core configuration. Processor utilization is based on several factors such as message rate, average message size, number of enabled transport agents, antivirus configuration, and third-party applications.

Exchange 2007 architecture has moved most of the client-specific functions from the Mailbox server to the Client Access server. In Exchange 2007, messages are converted on the Client Access server when they are accessed by a non-MAPI client (for example, POP3 and IMAP4 clients). In addition, rendering for Microsoft Outlook Web Access is performed on the Client Access server, as opposed to the Microsoft Exchange Information Store service in previous versions of Exchange Server. These architectural changes allow the Client Access server to offload significant processing from the Mailbox server and enable the Client Access server to effectively utilize 4 x processor cores. Servers with either 1 x or 2 x processor cores can be utilized for Client Access servers in organizations where there are not enough mailboxes or insufficient non-MAPI client traffic to warrant using 4 x processor core servers.

The recommended configuration for the Unified Messaging server role is 4 x processor cores. Multiple cores are used on the Unified Messaging server for several architectural functions such as .wav to Microsoft Windows Media Audio (WMA) conversions for voice mail messages. Servers with either 1 x or 2 x processor cores can be used for Unified Messaging servers in organizations where there are not enough mailboxes or insufficient Unified Messaging server activity to warrant using 4 x processor core servers.

The recommended configuration for the Mailbox server role is based predominantly on mailbox count and user profile. A 4 x processor core server provides a good balance between price and performance, and should be able to host several thousand mailboxes. Sizing for the Mailbox server requires an understanding of the average client user profile. This profile can be collected using the Microsoft Exchange Server Profile Analyzer or by using third-party tools. For more information, see the Microsoft Exchange Server Profile Analyzer Web site. The following table lists generic and common knowledge worker profiles for Microsoft Outlook clients.

Knowledge worker profiles for Outlook users

There are several factors to consider when sizing Mailbox servers other than the user types listed earlier. These include Mailbox servers that use Exchange 2007 features such as local continuous replication (LCR), and Mailbox servers that are accessed by Microsoft Forefront Security for Exchange Server, third-party applications, mobile devices, and online (versus Cached Exchange Mode) Outlook clients. Sizing used primarily for budgeting purposes can be accomplished by assuming that 1,000 average mailboxes will require a 1 x processor core. (For example, a 4,000-mailbox server with an average usage profile requires 4 x processor cores). A heavy usage profile effectively doubles the required processor cycles (or halves the number of users per processor core to 500 mailboxes per processor core). A 2,000-mailbox server with an average profile requires a 2 x processor core server. The maximum number of processor cores efficiently used by the Mailbox server role is eight. Deploying mailboxes on servers with more than eight cores will not provide significant scalability improvements.

In an LCR environment, both the active copy and passive copy of LCR-enabled storage groups are on the same server. In this environment, there is additional processing overhead generated from the Microsoft Exchange Replication service copying and replaying logs to the passive copy of the database. This additional processing overhead is approximately 20 percent and should be considered when sizing Mailbox servers that have one or more storage groups enabled for LCR.

The guidance for a computer with multiple server roles installed is similar to the Mailbox server role. To accommodate the Client Access and Hub Transport server roles on the same server as the Mailbox server role, reduce the 1,000 mailboxes per core calculation based on the average client profile by 20 percent (800 mailboxes per core) when performing sizing. The maximum recommended processor core configuration is listed at 4 x processor cores for the multiple server roles configuration to indirectly provide guidance on the maximum number of users that should be hosted on a multiple role server. Neither cluster continuous replication (CCR) nor single copy clusters (SCC) support hosting the Hub Transport or Client Access server roles in a failover cluster, so the multiple role server is non-clustered. It is a good idea to cluster Mailbox servers that host thousands of mailboxes to make sure that server maintenance or failures do not have a significant impact on uptime or availability. For this reason, the recommended maximum processor core configuration for the multiple server roles configuration is listed at four. Although this configuration can use up to eight processor cores, we do not recommend this configuration due to availability concerns.