Planning Server Role Ratios
Microsoft Exchange Server 2007 will reach end of support on April 11, 2017. To stay supported, you will need to upgrade. For more information, see Resources to help you upgrade your Office 2007 servers and clients.
Applies to: Exchange Server 2007, Exchange Server 2007 SP1, Exchange Server 2007 SP2, Exchange Server 2007 SP3
After you have determined your optimal processor, memory, and disk configurations, you should determine how many server roles of each type are required for your deployment. Every environment is different, so consider these recommendations as starting points that can be tailored to your environment.
These recommendations are based on the Microsoft internal deployment of Microsoft Exchange Server 2007. The following table details the characteristics of the Microsoft messaging environment. For more information about the Microsoft messaging environment, see Going 64-bit with Microsoft Exchange Server 2007.
Characteristics of the Microsoft messaging environment
|Heavy to very heavy
Primary client (weekday working hours)
Microsoft Office Outlook 2007 or Office Outlook 2003 Cached Exchange Mode (MAPI/RPC)
Primary after hours and weekend clients
Outlook 2007 or Outlook 2003 Cached Exchange Mode (Outlook Anywhere) and Outlook Web Access
Percentage of user base using Exchange ActiveSync
The following table shows recommended server role ratios that are based on the processor core guidance in the topic Planning Processor Configurations because it is common for server roles to have vastly different processor core counts. Also, the Mailbox server role is the basis for the processor core ratios. Hub Transport and Client Access server roles relate to the Mailbox server role with regard to the recommendation.
Ratings available at https://www.spec.org may be used to rationalize unlike processor and server configurations. The third-party Web site information in this topic is provided to help you find the technical information you need. The URLs are subject to change without notice.
Recommended server role ratios based on processor core
|Server role ratio
|Recommended processor core ratio
7:1 (no antivirus scanning on Hub)
5:1 (with antivirus scanning on Hub)
When considering these recommendations, be aware of the following:
The preceding ratios are a general rule, and they may not be valid for every topology. A general rule means that the ratios are not a requirement for support or a definitive rule.
Ratios can change dramatically based on user profiles. A user that creates a larger than expected load against the Mailbox server role than the Hub Transport server role will increase the Mailbox:Hub ratio, and vice versa.
These recommendations are derived from the internal deployment of Mailbox servers at Microsoft, which is based on approximately 500 heavy users per processor core.
These ratios assume that Mailbox servers are at greater than 60 percent processor utilization during peak periods, with corresponding processor utilization on Hub Transport or Client Access servers.
For these recommendations, the processors used on Mailbox, Hub Transport, and Client Access server roles were the same type and speed.
A minimum of two Hub Transport and two Client Access servers should be deployed for redundancy and to ensure uninterrupted service in case of planned or unplanned server downtime.
You can use the Exchange 2007 Management Pack for Microsoft Operations Manager 2005 Service Pack 1 (SP1) and the Performance Troubleshooter in the Toolbox node of the Exchange Management Console to determine if a specific deployment requires additional server roles based on performance. These tools can also be used to fine tune server role ratios for a specific deployment. For more information about the Exchange 2007 Management Pack, see Monitoring Exchange 2007 with Microsoft Operations Manager 2005 SP1. For more information about the Performance Troubleshooter, see Using the Toolbox.
The Hub Transport server role ratio that includes antivirus scanning was obtained using Microsoft Forefront Security for Exchange Server with five active scanning engines.
The Client Access server role ratio includes using Secure Sockets Layer (SSL) for all access protocols.
It is not possible to provide a ratio for the Unified Messaging server role because its utilization is not directly tied to the Mailbox server role. For more information about Unified Messaging server capacity planning, see Determining the Number of Users an Exchange 2007 Unified Messaging Server Can Support.
Edge Transport Server Count
To determine how many Edge Transport servers are required, you must measure or estimate the following metrics during peak periods:
Average message size
Sizing is based on the number of connections and messages processed, with average message size being a secondary factor. Because every Simple Mail Transfer Protocol (SMTP) connection does not translate into an SMTP message, and because every accepted message will not survive antivirus and anti-spam scanning, it is difficult to provide a simple sizing methodology based on message rate. Edge Transport server utilization depends on several factors that are unique to each organization.
A minimum of two Edge Transport servers should be deployed for redundancy and to ensure uninterrupted service in case of planned or unplanned server downtime.
The following table provides performance data values for key metrics from internal deployment at Microsoft. You can use the metrics and their values to further your understanding of the performance characteristics of an Edge Transport server.
Performance metrics from internal deployment of Edge Transport servers at Microsoft
% Connections Accepted
SMTP Messages IMF Scanned/Sec
% SMTP Messages passed IMF Scanning
SMTP Messages A/V Scanned/Sec
Avg. Message Size
** System included a 2-socket, dual-core AMD Opteron 275 2.2 gigahertz (GHz) processor
A significant percentage of the server processing can be associated with the overhead of analyzing connections and scanning accepted messages. For this reason, it is not possible to provide a sizing metric based solely on the number of messages sent and received per second because antivirus and anti-spam are significant processor utilization functions of the Edge Transport server role.
Active Directory Server and Mailbox Server Ratios
The recommended number of Active Directory directory servers in each site containing Exchange 2007 Mailbox servers or users depends on the number of processor cores in each computer running the Exchange 2007 Mailbox server role and the hardware platform on which Active Directory is running. Specifically, consider the following scenarios:
If Active Directory is running on the x86 platform (32-bit), the recommended ratio of Active Directory directory server processor cores to Exchange 2007 Mailbox server processor cores is 1:4.
If Active Directory is running on the x64 platform (64-bit), the recommended ratio of Active Directory directory server processor cores to Exchange 2007 Mailbox server processor cores is 1:8. To achieve the 1:8 ratio, you must have enough memory installed on the directory server to cache the entire Active Directory database in memory. To check the size of your Active Directory database, examine the NTDS.DIT file on a global catalog server. By default, this file is located in %WINDIR%\NTDS.
In the above ratios, it is important to note that this is a ratio of processor cores and not processors. Thus, a dual-core processor counts as 2 when calculating the ratio.
For Exchange 2007, we recommend that you deploy one 32-bit global catalog (GC) server processor core for every four Exchange 2007 Mailbox server processor cores. Although other server roles will influence the number of GC processor cores required, the Mailbox servers that are deployed influences the deployment of each of the other roles, so basing the number of GC processor cores on Mailbox server processor cores will suffice.
For additional guidance about Active Directory directory server sizing and ratios, see Guidance on Active Directory design for Exchange Server 2007.