At firs you should understand what is VM migration and why do we need to migrate VMs. At what circumstances a VM should be migrated.

The main reason to migrate a VM is when ""cold spot" and "hot spot" happens. Do some self reading on these stuff.

Then only you will come across the question of VM placement

You can't certainly say what the local optima is, but you can say that your system works at its best when after VM placement, you won't face the issues of hot/cold spot again.. Otherwise you have wasted the resources..

Resources in this case would be an inefficient bandwidth utilization, extra time and overhead, delay and etc

So, there are few factors that you must take into consideration about VM migration:

When to migrate, Which VM to migrate and where to migrate... the last one addresses your question more/less

You should come up with an algorithm that can effectively determine where is the most optimal place you should migrate your VM to, based on the resource availability and affinity.

Hope that can answer your question. 

Hi dear Mr.Maleki ! Thanks for your thorough explanation ! So, for VM placement I should simulate the whole 4 steps of server consolidation and apply my multi-objective algorithm in the 4th part which is VM placement ?

Why do u need server consolidation at first place ?

You should first look into that due to what issue you want to migrate the VM and WHERE do u want to migrate it. Server consolidation will not necessary help... Why not load balancing ? You must answer the following question first.. Your algorithm must address these issues

When to migrate?

Which VM to migrate?

Where to migrate the VM?

yes! exactly!I should answer these questions first ! But server consolidation is an approach for resource management which leads to energy consumption minimization and resource wastage ! That's why I wanna use it ! And my placement algorithm cares about these 2 objectives.