Which Servers Are Not Good Virtualization Candidates?
Almost all servers and workloads can be virtualized, but in some cases you may not want to virtualize certain servers because of high-resource utilization, licensing issues, and application support issues. Let's cover some reasons you might not want to virtualize certain servers and some reasons that you may consider virtualizing these types of servers:
High-resource utilization servers
Why you might not want to virtualize. A server that has very high resource requirements may not always be as good a fit as a virtual server. Typically, these types of servers have very high CPU and memory usage and high disk and network I/O, and on a virtual host where multiple servers are competing for resources they might not perform as well.
Why you should consider virtualizing. When virtualizing these types of servers, you may be able to have only one or two VMs on a host server. You might wonder why anyone would want to put just a single VM on an ESX host. The reason for this is to take advantage of some of the powerful features that virtualization offers such as snapshots, VMotion, and high availability (HA) that are more difficult and costly to implement in a physical environment. Also, virtualizing these servers can make for easier disaster recovery implementation and simplified hardware upgrades.
Vendor licensing models
Why you might not want to virtualize. Some applications, such as Oracle, do not have virtualization-friendly licensing and require you to license their software based on the number of physical CPUs in the host server and not the number of virtual CPUs assigned to the VM that is running the application. So, a VM with two virtual CPUs on a four-CPU host server would require you to purchase a license for four CPUs regardless of the fact that the VM that is running the application has only two virtual CPUs.
Why you should consider virtualizing. Thankfully, most vendors today license their software based on the number of CPUs assigned to the server regardless of whether they are physical or virtual. Other vendors are changing their licensing models to meet the growing demand for using virtual servers. Check with your vendor to see if they have changed their licensing model or have any plans to do so in the future.
Why you might not want to virtualize. Some vendors will not provide support for their application if it is running on a virtual server. We discuss this in more detail later on.
Why you should consider virtualizing. Very few vendors do not support running their applications on virtual servers. Consider alternative support options. (For example, HP provides support for both VMware and Microsoft products.)
Specific licensing requirements:
Why you might not want to virtualize. Certain applications use stricter licensing methods to prevent piracy and illegal use of their software. Examples of this are hardware dongles (parallel/serial port/USB device keys) that plug into the server and must be present at all times and specific MAC address or hard drive serial number licensing. If a VM moves from one host to another as a result of a failure or due to resource constraints on a host, then the hardware dongle will no longer be present and the application will no longer work.
Why you should consider virtualizing. There are ways to accommodate these types of licensing schemes on virtual servers. Digi makes a device called AnywhereUSB that works with ESX servers and provides IP-based connections to USB devices.
Hardware that cannot be virtualized
Why you might not want to virtualize. Some servers might have nonstandard hardware like fax and modem boards that are not supported in ESX which supports only a limited, very specific set of hardware.
Why you should consider virtualizing. Solutions are available for faxing and using modems via network connections over IP.