Simply put VM sprawl is the uncontrolled growth of virtual machines in a virtual environment. Star Trek fans can relate to this from the popular “Trouble with Tribbles” episode where the tribbles reproduce so quickly that they threaten to overwhelm the host ship's food supplies. VM sprawl is similar because many administrators create VMs without any regard for the resources that they consume and the possibility of overwhelming the host server’s resources. Many consider VMs as “free” servers since VMs do not have a physical presence and creating them is simple and easy.
Some ways to deal with sprawl are implementing a chargeback system like Vkernel’s capacity and chargeback appliance or Vizioncore’s vCharter products for VMware that create reports on resource usage. Microsoft Hyper-V will have some charge-back metrics available but lacks the tools to make effective use of them. In addition limiting who can create virtual machines and implementing a formal process for requesting new virtual machines is a more effective method then allowing too many people to have access to create virtual machines at will. You should require justification for requests for any new VMs and have an approval process to get users to think twice about if they really need another VM. Finally creating resource pools is a helpful way to to limit the amount of resources available on your host servers for new VMs.
If you don’t control sprawl you may use up all your host resources before you know it and also create bottlenecks that may reduce the performance of all your VMs. Tight control of your virtual environment is the key to limiting the growth of virtual machines.