When Virtualization can inhibit hardware upgrades, firmware & OS upgrades
Enterprise mission critical applications requiring performance and availability should be isolated as much as possible from other application's maintenance including Hyper-Visor and OS patch updates as well as security requirements. Such mission critical applications should be run as close to machine residence as possible and isolated from firmware, software, networking, and security layers requiring regular configuration changes including updates and patches.
Granted, with the advent of hot swappable hardware, dynamic firmware upgrades, and hot patching OS's without outages reduce the probability of a scheduled outage. Never the less, the more levels of virtualization complicate troubleshooting and increase the potential for unplanned outages. Virtualization also makes clustering more complex and adds still another layer of potential failure and complexity for memory and disk storage management, I/O subsystems, networking, and security.
Many application services may be virtualized and clustered but Enterprise mission critical applications should be isolated from as many layers of complexity as possible for reliability, serviceability, and availability.
