Sizing Client Access Servers
Applies to: Exchange Server 2007 SP3, Exchange Server 2007 SP2, Exchange Server 2007 SP1, Exchange Server 2007
Topic Last Modified: 2011-01-13
In Microsoft Exchange Server 2007, the sizing of Client Access servers is performed differently from the sizing of front-end servers in previous versions of Exchange. The architecture changes in Exchange 2007 have moved most of the client-specific functions from the Mailbox server to the Client Access server. For more information about the functions and features provided by the Client Access server, see Client Access.
In Exchange 2007, messages are converted on the Client Access server when they are accessed by a non-MAPI client (for example, Internet Message Access Protocol 4 (IMAP4) or Post Office Protocol 3 (POP3) 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, which rendered Outlook Web Access in previous versions of Exchange.
The Client Access server also offloads many stateless tasks from the Mailbox server (assuming the roles are installed on different physical servers), and provides a unified namespace so that users need only point to a single name regardless of which Mailbox server hosts their mailboxes. In addition to the Internet protocols, such as IMAP4, POP3, and HTTP, the Client Access server also provides Outlook Anywhere (formerly known as RPC over HTTP), ActiveSync, Autodiscover, Availability service (which includes all free/busy information in topologies that do not use public folders), and Web services. These architectural changes allow the Client Access server to offload significant processing from the Mailbox server.
In general, memory utilization on Client Access servers has a linear relationship with the number of client connections and the transaction rate. Based on the current recommendations for processor and memory configurations detailed in Planning Processor Configurations, Planning Memory Configurations, Planning Server Role Ratios, and Planning Storage Configurations, a Client Access server will be balanced in terms of memory and processor utilization, and it will become processor-bound at approximately the same time it becomes memory-bound.
The Client Access server can be affected by processor, memory, and network bottlenecks, yet it has a small disk input/output (I/O) footprint. Simple Mail Transfer Protocol (SMTP) traffic, a potential disk I/O consideration in front-end servers running Exchange Server 2003 and Exchange 2000 Server, is now associated exclusively with the Hub Transport servers and Edge Transport servers.
In Exchange 2007, the effect of high item count folders can be significant when you access those folders from an IMAP4 client. Though the general guidance for item count in Exchange 2007 suggests an upper limit of 20,000 items in critical path folders, such as the Inbox, item count in folders accessed by IMAP4 clients should be limited to 10,000 for the best possible user experience and for minimal impact on performance and scalability of the CAS role.
Lab testing has shown that the amount of processor capacity that is consumed on Client Access servers per message during the simple steps of opening a folder and retrieving the unique ID and flags associated with each message is relatively linear up to approximately 10,000 messages in a folder. This consumption increases significantly at higher item counts. For more information about folder item counts, see Understanding the Performance Impact of High Item Counts and Restricted Views.