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What are the disadvantages of server virtualization?

Implementation and licensing costs
Virtual server sprawl
Data backup
Single point of failure
Server security
Spinning up virtual servers is too easy
Resource contention
Performance problems
Education and training
Additional layer in the stack

Although server virtualization brings a host of potential benefits to the organization, the additional software and management implications of virtualization software bring numerous possible disadvantages that the organization should consider:

Risk and availability. Running multiple workloads on the same physical computer carries risks for the organization. Before the advent of virtualization, a server failure only affected the associated workload. With virtualization, a server failure can affect multiple workloads, potentially causing greater disruption to the organization, its employees, partners and customers. IT leaders must consider issues such as workload distribution -- which VMs should reside on which physical servers -- and implement recovery and resiliency techniques to ensure critical VMs are available in the aftermath of server or other physical infrastructure faults.

VM sprawl. IT resources depend on careful management to track the availability, utilization, health and performance of resources. Knowing what's present, how it's used and how it's working are keys to data center efficiency. A persistent challenge with virtualization and VMs is the creation and eventual -- though unintended -- abandonment of VMs. Unused or unneeded VMs continue to consume valuable server resources but only do a little valuable work; meanwhile, those resources aren't available to other VMs. Over time, VMs proliferate and the organization runs short of resources, forcing it to make unplanned investments in additional capacity. The phenomenon is called VM sprawl or virtual server sprawl. Unneeded VMs must be identified and decommissioned so that resources are freed for reuse.

Resource shortages. Virtualization makes it possible to exceed normal server resource utilization, primarily in memory and networking. For example, VMs can share the same physical memory space, relying on conventional page swap -- temporarily moving memory pages to a hard disk so the memory space can be used by another application. Virtualization can assign more memory than the server has; this is called memory overcommitment. Overcommitment is undesirable because the additional latency of disk access can slow the VM's performance. Network bandwidth can also become a bottleneck as multiple VMs on the same server compete for network access. Both issues can be addressed by upgrading the host server or by redistributing VMs between servers.

Licensing. Hypervisors and associated virtualization-capable management tools impose additional costs on the organization, and hypervisor licensing must be carefully monitored to observe the terms and conditions of the software's licensing agreements. License violations can carry litigation and significant financial penalties for the offending organization. In addition, bare-metal VMs require independent OSes, requiring licenses for each OS deployment.

Experience. Successful implementation and management of a virtualized environment depends on the expertise of IT staff. Education and experience are essential to ensure that resources are provisioned efficiently and securely, monitored and recovered in a timely manner, and protected appropriately to ensure each workload's continued availability. Business policies play an important role in resource use, helping to define how new VMs are requested, approved, provisioned and managed throughout the VM's lifecycle.

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What are the disadvantages of server virtualization?

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