High-resource utilization servers

When virtualizing these types of servers, you may be able to have only one or two VMs on a host server, but you can take advantage of some of the features that virtualization offers such as snapshots, VMotion, HA, disaster recovery, and data protection, that are more difficult and costly to implement in a physical environment.

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.

Vendor licensing models

Thankfully, most vendors today have specific license rules for virtual environments, and in some cases are also virtualization friendly (for example, with a Windows Server data center, you can run unlimited instances on a properly licensed host).

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.

Licensing restriction or limitation

There are some possible ways to accommodate these types of licensing schemes on virtual servers, such as USB redirection or specific network appliances (or Digi AnywhereUSB).

Certain applications use stricter licensing controls, such as hardware dongles (parallel/serial port/USB device keys), MAC address, or hardware serial controls.

Hardware that cannot be virtualized

Solutions are available for faxing and using modems through network connections over IP, but it's also possible to consider the PCI Passthrough feature of ESXi.

Some servers might have non-standard hardware such as a fax and modem.

Application support

Very few vendors do not support virtualization, but in those cases, you may consider keeping some physical server to reproduce the issue on them and still have the right support.

Some vendors will not provide support for their application if it is running on a virtual server.

Avoid too many eggs in the same basket

You can still consider different clusters (for example, a management cluster) or use proper QoS solutions.

Shared infrastructure could mean shared problems (for example, storage performance).

Dependency or risk prevention

You can still consider different clusters (for example, a management cluster) or a different site.

Sometimes you have a circular dependency that can be a risk in case of a major failure. Dependencies are also needed to understand how power-off or power-on the entire infrastructure.