A zombie VM is a virtual machine that is created, often for a particular purpose, that is forgotten when it is no longer needed for that purpose.
As a result, the VM continues to consume resources while performing no useful work. The proliferation of zombie VMs in a data center is one of the main causes of virtualization sprawl.
Benefits of virtualization include the ability to create virtual machines quickly and easily, along with the fact that each VM requires only a very small portion of the physical infrastructure's resources, which means that it is inexpensive to run and doesn't have a lot of impact on the consumption of storage and other resources. Most zombie VMs are simply temporary virtual servers that have been forgotten due to lax tracking procedures.
Ideally, the creation of each VM should be documented, along with its purpose and owner. When the VM is no longer required, it should be removed. Once virtualization sprawl has occurred, however, it may be necessary to search out the zombie VMs from the productive ones. Indications of a zombie VM include an absence of recent support tickets and low CPU, network and disk utilization.