Architecting a Hybrid Messaging Strategy with Microsoft Exchange 2013 : Designing the Solution : 7.2 Server Sizing : 7.2.1 DAG Process Requirements
   
7.2.1 DAG Process Requirements
The DAG feature in Exchange 2013 requires additional considerations when sizing the mailbox server role for a customer design, forcing the architect to account for both active and passive mailboxes. As we have seen in this design, when a mailbox server is a member of a DAG, that virtual server can also host one or more passive databases in addition to any active databases for which they might be responsible. When calculating compute resources, note that each passive database is likely to add an additional 10 percent to the CPU requirements of the mailbox server already hosting an active copy of a mailbox database.
 
The following figure illustrates this principle. In the figure, there are three Microsoft Exchange mailbox servers, each with an active database (MSDB1a denotes database 1 active) and two passive databases from the other two mailbox servers (MSDB1p denotes database 1 passive). Each passive copy of MSDB1a requires 10 percent extra processing on the server hosting MSDB1a, for a total of 20 percent extra CPU overhead.
Figure 6. DAG Layout with Overhead for Passive Databases
 
Therefore, each mailbox server in the sample architecture requires an additional 20 percent of processing power to account for passive database copies. The sizing process begins with understanding and applying Microsoft guidelines for each server role, as represented by the following high-level processes:
• Design the mailbox server DAG nodes:
o Apply Microsoft guidelines to determine CPU and memory requirements. Considerations include number of mailboxes, mailbox profile, number of servers in the DAG, number of passive database copies, and several other custom parameters.
o Use the Exchange 2013 Mailbox Server Role Requirements Calculator (https://gallery.technet.microsoft.com/Exchange-2013-Server-Role-f8a61780) from Microsoft to determine storage requirements. (Third-party Web sites are not under the control of VMware, and the content available at those sites might change.)
• Design the peripheral server roles:
o The Exchange 2013 Mailbox Server Role Requirements Calculator also recommends CPU and memory for the CAS and ETS roles.
• Allocate one or more virtual machines for each server role to satisfy the previously calculated number of processor cores and amount of memory.
• Determine how the virtual machines will be distributed across ESXi hosts on site and the resources required for the provider environment.
• Aggregate virtual machine requirements plus some overhead to size each ESXi host. This overhead is important if you want to minimize the performance impact during the loss of an ESXi host. A typical guideline when choosing the number of required hosts is n+1, where n is the number of hosts required to run the workload at peak use. n+1 allows you to design for the possibility of losing one host from your VMware cluster without taking a performance hit during failover.