For as long as server virtualization has been popular, administrators have had to decide what type of server should and should not be virtualized. Determining whether to virtualize a certain type of server has been the subject of many spirited debates among IT pros and on various Internet message boards. Unfortunately, there is no clear-cut answer that holds up in every situation. However, there are guidelines that you can use to determine if a server is a good candidate for virtualization.
Hypervisor support and limitations
One of the first criteria that I recommend taking into account is supportability. Some servers simply are not supported for use in a virtual environment. For example, Microsoft does not support running Exchange Server 2007’s Unified Messaging role on a virtual server, per the support policy and recommendations for Exchange Server 2007.
Normally, there is a very good reason if an application’s official support policy expressly prohibits running the application on a virtual machine (VM). However, this does not always hold true. There are organizations that still run business critical applications on Windows NT Server. Windows NT Server is not supported for use with Hyper-V. However, this does not mean that Windows NT Server will not work with Hyper-V. Microsoft does not support running Windows NT on a Hyper-V VM simply because the operating system is so old that it is not supported at all. Virtualizing a Windows NT Server can provide a number of benefits, including increased reliability and performance.
The decision on whether to virtualize a particular type of server often comes down to hypervisor limitations. Some physical servers are equipped with hardware that simply exceeds a hypervisor’s capabilities. For example, Windows Server 2008 R2 Hyper-V supports a maximum virtual hard disk size of 2 TB. There are plenty of physical servers in the world with volumes exceeding 2 TB. Such servers would not be good candidates for virtualization if Windows Server 2008 R2 Hyper-V were the hypervisor. Incidentally, the Windows Server 2012 version of Hyper-V extends the maximum virtual hard disk size limit to 64 TB with the new VHDX virtual hard disk format.
Keep in mind that virtual hard disk size is not the only hypervisor limitation that could potentially affect your ability to perform a physical to virtual (P2V) conversion. Limitations on supported memory or a maximum number of virtual processors could also make a difference.
Financial implications of virtualizing
Assuming that you have all the necessary licenses for your physical server, there may not be any direct costs associated with virtualizing. Even so, there are situations in which performing a P2V migration could prove cost prohibitive.
Imagine, for example, that your host servers lack the capacity needed to handle the new VM. In this situation, you would probably have to either perform a major upgrade on the host or purchase a new host server. Both options could be expensive, and it may be hard to justify the cost.
Of course, host servers are almost always part of a cluster, which can complicate resource requirements. For example, if the cluster itself is running at near maximum capacity, there might be room to accommodate the newly migrated server, but you might not have enough remaining capacity to allow that VM to fail over to another cluster node in the event of an emergency.
Similarly, you could run into a situation in which accommodating the new VM means performing a hardware upgrade on each node in the cluster, which could be expensive. Imagine a physical server that connects to storage area network (SAN) storage. Traditionally, many people have not considered this type of server to be a good candidate for virtualization. However, the Windows Server 2012 version of Hyper-V allows VMs to connect directly to SAN storage through virtual Fibre Channel. However, this connection requires that the host server be equipped with one or more Fibre Channel Host Bus Adapters.
Depending on the type of Host Bus Adapters you need, you might be able to outfit the virtualization host without breaking the bank. However, if that host server is part of a cluster, then you will also need to purchase Host Bus Adapters for any other node in the cluster to which the VM could potentially fail over.
As you can see, there are a number of different criteria that can determine whether a physical server is a good candidate for virtualization. Often times the decision-making process comes down to assessing the practicality and the cost of the migration.