Understanding Server Role Ratios and Exchange Performance

Exchange 2010

Applies to: Exchange Server 2010 SP3, Exchange Server 2010 SP2

Topic Last Modified: 2010-01-26

After you've 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.


Server Role Ratios

Edge Transport Server

Active Directory Server and Mailbox Server Ratios

The following table shows recommended server role ratios that are based on the processor core guidance in the topic Understanding Processor Configurations and Exchange Performance. 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.

You can also use the ratings available at Standard Performance Evaluation Corporation to rationalize different processor and server configurations.

Recommended processor core ratio based on server role configuration

Server role configuration Recommended processor core ratio

Mailbox:Hub Transport

7:1 (no antivirus scanning on Hub Transport server)

5:1 (with antivirus scanning on Hub Transport server)

Mailbox:Client Access


Mailbox:Client Access and Hub Transport combined role


The recommended Mailbox to Client Access server role ratios are based on the use of the Exchange 2010 feature, RPC Client Access. This feature requires a larger memory and processor configuration to manage the increased loads placed on the Client Access server role. The RPC Client Access feature is enabled by default. For more information, see Understanding RPC Client Access.

When considering these recommendations, be aware of the following:

  • The preceding ratios are a general rule (not definitive), and they may not be valid for every topology. A general rule means that the ratios aren't a requirement for support.

  • Ratios can change dramatically based on user profiles. A user that creates a larger than expected load against the Mailbox server role rather than the Hub Transport server role will increase the Mailbox:Hub Transport ratio, and vice versa.

  • These recommendations are derived from the internal deployment of Mailbox servers at Microsoft, which is based on approximately 750 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.

  • 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's not possible to provide a ratio for the Unified Messaging server role because its utilization isn't directly tied to the Mailbox server role. For more information about Unified Messaging server capacity planning, see Understanding Unified Messaging Availability.

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To determine how many Edge Transport servers are required, you must measure or estimate the following metrics during peak periods:

  • Connections per second

  • Messages per second

  • Average message size

Sizing is based on the number of connections and messages processed, with average message size being a secondary factor. Because every SMTP connection doesn't become an SMTP message, and because every accepted message won't survive antivirus and anti-spam scanning, it's 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 help you understand the performance characteristics of an Edge Transport server.

Performance metrics from internal deployment of Edge Transport servers at Microsoft

Performance metric Value

SMTP Connections/Sec


% Connections Accepted

80 %

SMTP Messages IMF Scanned/Sec


% SMTP Messages passed IMF Scanning

80 %

SMTP Messages A/V Scanned/Sec


Avg. Message Size

70 KB

CPU Utilization

20 %**

** System included a 2-socket, dual-core AMD Opteron 275 2.2 gigahertz (GHz) processor

A significant percentage of the server processing is associated with the overhead of analyzing connections and scanning accepted messages. For this reason, it's not possible to provide a sizing metric based solely on the number of messages sent and received per second because antivirus and anti-spam operations are significant processor utilization functions of the Edge Transport server role.

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The recommended number of Active Directory directory servers in each site containing Exchange 2010 Mailbox servers or users depends on the number of processor cores in each computer running the Exchange 2010 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 2010 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 2010 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 preceding ratios, it's 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. The ratio difference between 32-bit and 64-bit is due to the larger amount of memory that a 64-bit operating system can support as compared to a 32-bit operating system.

For Exchange 2010, we recommend that you deploy one 32-bit global catalog server processor core for every four Exchange 2010 Mailbox server processor cores, or one 64-bit global catalog server processor core for every eight Exchange 2010 Mailbox server processor cores. Although other server roles will influence the number of global catalog processor cores required, the Mailbox servers that are deployed influences the deployment of each of the other roles, so basing the number of global catalog processor cores on Mailbox server processor cores will suffice.

For additional guidance about Active Directory directory server sizing and ratios, see Planning Active Directory.

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