The cost benefit of getting as many as 10 or 20 virtual servers for the price of one physical box drove many companies to migrations that covered as much as 25 percent to 35 percent of all the servers targeted for conversion, before hitting "VM stall," a virtual halt in migrations caused by the most subtle cost- and organizational issues that affect virtualization projects directly, according to James Staten, principal analyst at Forrester research. "Companies can get close to the 50 percent point [in a P2V migration] still using the same thinking they did in the physical world," Staten says. "Obvious costs like licenses, how many machines you can take out of an environment, how many VMs you can put on a host all make one cost picture. Beyond that you get into issues about performance and capacity management, and the amount of effort needed for support -- a lot of companies don't take those fully into account."

Planning to virtualize every workload on every server without modifying the way IT plans capacity requirements or the way it allocates computing resources and IT staff support time, leaves IT departments with a lot of duplicated processes -- and a steadily dropping return on investment as a P2V migration expands, says Chris Wolf, research VP at Gartner.

"Trying to replicate the same structures you had been using, with virtual servers, gets into a cycle of diminishing returns pretty quickly," Wolf says. Keeping virtualization projects on track requires changes in both organization and technology -- and the need to keep the two coordinated according to the particular stage of migration, Staten says. Here's some advice for avoiding stalls during four key phases of a virtualization project.

The first, ecstatic wave of virtualization saves far more money, far more quickly than at any other time during the migration to or operation of a virtual infrastructure, according to Gary Chen, research analyst at IDC.