Capacity planning for Exchange Server has always been a bit of a catch-22. Microsoft offers tools such as Microsoft Exchange Server 2003 Load Simulator (LoadSim) and Microsoft Exchange Server Jetstress Tool to help you determine how well your Exchange server will handle an anticipated workload. But they aren't true capacity-planning tools because you can run them only after you have the new server in place—or at least have a hardware configuration similar to what you plan to deploy.
These tools are great if you want to find out whether a new Exchange server can comfortably handle the anticipated workload before you actually set up users' mailboxes. However, they do nothing to help you figure out what hardware to buy. What happens if you spend thousands of dollars on a new Exchange server only to run these tools and find out that the server won't be able to get the job done? Even if it were possible to run a basic planning tool such as LoadSim before purchasing new hardware, the results require evaluation and therefore can be misinterpreted.
Last year, Microsoft released its first true capacity-planning tool for Exchange Server, Microsoft System Center Capacity Planner 2006. You can use this tool to plan for either a new Exchange server or for deploying an entire Exchange organization.
You can't buy Capacity Planner directly, or download it from Microsoft's Web site. Instead, the tool is available only through a Microsoft Developer Network (MSDN) or TechNet subscription. However, you can learn more about the tool and its capabilities on Microsoft's System Center Capacity Planner 2006 Web site at http://www.microsoft.com/systemcenter/sccp/default.mspx. Capacity Planner also models Microsoft Operations Manager (MOM) 2005 deployments, though MOM is not a requirement for using the tool for Exchange modeling. Let's take a look at Capacity Planner and how to configure it to model an Exchange deployment. (Note: In October 2006, Microsoft released System Center Capacity Planner Service Pack 1—SP1— which makes the tool Windows Vista compatible, fixes a few bugs, and adds support for low-speed WAN links. The real enhancements in SP1 are related to MOM, not to Exchange.)
Basic Steps
There are three main steps involved in using Capacity Planner. The first step in the process is to create a model of your network (or of the network you're contemplating building) by using Capacity Planner's Model Wizard. As you answer questions about your network, the wizard creates a model of your proposed network, including a graphical representation. When the model is complete, you can run a simulation on your proposed Exchange environment.
The important thing to remember about the Model Wizard is that it's intended only as a starting point. The wizard saves you the tedious chore of creating a sample network manually by generating a model that at least somewhat resembles the network you're considering building. However, in almost every situation, you'll have to do additional work on the model after the wizard finishes.
The reason for the extra work is that the Model Wizard is grossly oversimplified. For example, at one point in the process, the wizard asks what type of disk configuration your servers use. Whatever configuration you select, the wizard will assume that every server in your Exchange organization uses the same configuration.
The second step in the capacity-planning process is to use the Model Editor to revise the model you created with the Model Wizard so that it accurately reflects your network (or proposed network). Revising the model includes doing things such as adjusting the CPU type for individual servers and setting the speed for various WAN links.
After you customize your model, it's time for the third step: running the simulation. This is where you find out whether the Exchange server or servers that you're planning to deploy will be able to handle the anticipated workload. The nice thing about the simulation is that it's dynamic: The simulation allows you to analyze various "what if" scenarios.
Although Capacity Planner is a great tool, it isn't perfect. You shouldn't base hardware-purchasing decisions solely on what the simulation tells you. Most of the time, this tool will be fairly accurate. But you also have to use common sense. Stop and think about whether the information that Capacity Planner is giving you seems reasonable.
One reason the simulation might not be completely accurate is that it assumes your Active Directory (AD)—which Exchange Server depends on—is configured optimally. If your domain controllers (DCs), Global Catalog (GC) servers, or DNS servers are acting as a bottleneck, the results will be skewed. The simulation is also not completely accurate because it doesn't take into account the overhead that antivirus or antispam software consumes.