When virtualizing Exchange Server 2010, it’s important to correctly configure virtual machines (VMs) that will host certain Exchange Server roles – otherwise a few gotchas will surface. For example, you’ll need to assign the correct number of processors to the appropriate amount of RAM. This tip gives more advice for correctly configuring VMs for optimal performance.
Client access and hub transport servers
Even in small Exchange environments, the Client Access Server role and the Hub Transport Server role are commonly collocated with the Mailbox Server role on a single computer. For organizations with hundreds or thousands of mailboxes, this is an all-in-one configuration.
Resource requirements for the CAS are usually less than what the hub transport server traditionally needs. Since every piece of mail in an Exchange Server 2010 organization flows through the hub transport server — for routing and messaging policy application purposes — it bears a heavy load.
Microsoft states that each role should be given a minimum of 2 GB of RAM, with a recommended setting of 1 GB per core for the hub transport server and 2 GB per core for the client access server. Comparative requirements for the client access server can be larger in environments that heavily rely on Outlook Web App (OWA).
As a rule, virtualization environments work best when VMs are configured with the smallest number of processors possible. The processor power requirements of your Exchange 2010 environment will determine that number. Starting small and working up is the best rule of thumb.
Microsoft suggests eight cores for the hub transport server and four cores for the client access server to support an organization of several mailbox servers and thousands of mailboxes. Organizations with fewer mailboxes and lower levels of non-MAPI client traffic could start with as few as two cores.
Edge transport servers
Edge transport servers typically consume lower levels of resource utilization. This level can depend on the rate of inbound and outbound mail flowing through your Exchange organization, along with your level of rejected mail. If your organization deploys spam filtering from an upstream provider, this reduction in traffic can affect edge transport resource utilization.
Microsoft also suggests configuring edge transport servers with a single processor core and up to a maximum of twelve cores — if your virtual platform will support them. RAM requirements start with a minimum of 2 GB with 1 GB per core. You can always add more memory later if you experience excessive paging, poor performance or an email build-up in messaging queues.
Unified messaging servers
This is easy — don’t virtualize unified messaging servers. Microsoft does not support them. Unified messaging servers require substantial processing power and have little tolerance for processing latency. Microsoft’s suggestion for even a small Exchange organization is a minimum of two to four cores and 4 GB of RAM, with 2 GB per core of RAM. If you’re using unified messaging, I advise that you steer clear of virtualization for now.
Mailbox servers
Mailbox servers are the heavy-lifters in any Exchange infrastructure; they are where resources are used in larger quantities. You might want to consider virtualizing mailbox servers last. While they can be virtualized, they require careful evaluation before beginning. Experience gained from virtualizing Exchange’s other less-challenging roles will help.
Microsoft states that a four-core mailbox server should be able to support several thousands of mailboxes. The RAM recommendation for that server starts with 4 GB of RAM plus an additional 3 to 30 MB per mailbox. Virtualizing any mailbox server will consume a large share of your virtual host’s available RAM resources, so plan accordingly and don’t oversubscribe your virtual infrastructure.
Exchange Server 2010 virtualization gotchas
Exchange Server organization’s mail usage characteristics have more to do with these calculations than any RAM or core numbers suggested here. Measuring your existing metrics on physical servers is the first step in preparing to virtualize Exchange Server. Measured over time, those metrics are necessary to determine your starting points for processor and memory resource assignment.
Microsoft states that relying on someone else’s numbers is a flawed assumption. In Understanding Server Role Rations and Exchange Performance, Microsoft notes, “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….”
While this quote relates specifically to the activities within the edge transport server, it’s also good advice for the other roles. If you don’t correctly determine a baseline for your Exchange 2010 server role performance before you begin a virtualization project, any or all of the roles may cause trouble.
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