One of the biggest selling points of Exchange server virtualization is that it consolidates server workloads, allowing you to use your hardware resources more effectively. While this philosophy sounds simple, the actual consolidation process is something of an art form.
Exchange Server 2007 can be virtualized, but each individual Exchange server has needs that must be taken into account. You can't just virtualize your entire Exchange Server organization, place all of your virtual Exchange servers onto a single host and expect it to work well. The process of consolidating virtual machines (VMs) requires planning.
The first step in virtualizing or consolidating an Exchange organization is to take inventory of your existing Exchange servers. As you do, be sure to document the server's hardware configurations and which server is assigned to each Exchange Server role. Even if you have a good idea of how your Exchange organization is configured, it's important to know exactly how each server is configured.
Once you've taken inventory of your hardware for each physical Exchange 2007 servers, it should be easy to begin creating the VMs that will replace them. Most hypervisor-based virtualization platforms make it easy to allocate host resources like memory and hard disk space. But there are a few guidelines to keep in mind as you allocate hardware resources.
Microsoft does not provide a separate set of system requirements for virtualized Exchange servers. The system requirements are identical regardless of whether Exchange Server is running on a dedicated server or within a VM.
It stands to reason that if your current hardware does a good job running Exchange Server, then allocating equivalent resources on the virtualization host should produce comparable results. However, this is only partially true.
A virtual machine's performance may not be identical to Exchange server hardware performance because the virtualization software causes a small amount of overhead. This overhead, however, usually doesn't cause any major performance issues.
One factor that contributes more heavily to performance inequality is contention of resources. The host has a finite pool of resources that all VMs and the host OS (if used) share. Some virtual machines can only reserve certain resources.
For example, it is possible to dedicate a specific amount of memory to a VM. In doing so, that amount of memory is inaccessible to other virtual machines and to the host OS. This memory allocation does not work with other types of resources, though.
Consider, for example, the server's disk resources. Although virtualization software will allow you to reserve disk space for a VM, you can't reserve disk time. This means that if all of your virtual hard drives are on the same disk array, various virtual machines will all try to access that array simultaneously.
If the array does not provide sufficient bandwidth to accommodate all of those requests in a reasonable amount of time, the server's performance will suffer. This is especially true for mailbox servers, which tend to be much more I/O intensive than other Exchange server roles.
It can also be difficult to allocate CPU cores. While most virtualization products will allow you to allocate virtual processors, these processors do not always scale as they would on a physical server because processor utilization is relative to the resources that the host OS and other VMs use.
For example, if you dedicate a virtual processor to a VM, that virtual processor may be asked to perform other duties outside of the virtual machine. This would prevent the processor from performing as well as if the server was running on dedicated hardware.
Note: It's pretty easy to create virtual machines that match existing Exchange server performance. I recommend running the System Center Capacity Planner before you begin. Making some small adjustments to your planned hardware allocation may improve performance. Although System Center Capacity Planner isn't really designed for use in virtual data centers, it does offer valuable insight about how Exchange will perform with various hardware configurations.
About the author: Brien M. Posey, MCSE, is a five-time recipient of Microsoft's Most Valuable Professional (MVP) award for his work with Exchange Server, Windows Server, Internet Information Services (IIS), and File Systems and Storage. Brien has served as CIO for a nationwide chain of hospitals and was once responsible for the Department of Information Management at Fort Knox. As a freelance technical writer, Brien has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal website at www.brienposey.com.
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