Достаточно интересный документ от "DAVID DAVIS" и "BRAD BONN" при финансовой поддержке компании "VKernel" (проще говоря, спонсора). Вот описание, взятое из данного документа:

When sizing virtual machines a virtualization administrator must select the number of vCPUs, the size of the virtual disk, the number of vNICs, and the amount of memory. Out of all those resources, the amount of memory allocated to each virtual machine (VM) is often the most difficult to determine. This is because memory is the most dynamic and least predictable of those resources.

Virtualization administrators should avoid over-allocating memory to their VMs because doing so can waste expensive server resources and will, therefore, minimize the return on the investment (ROI) in that infrastructure. On the other hand, the business-critical applications running on virtual machines also need to maintain high performance, and the last thing a virtualization administrator wants is to have performance complaints from end users.  

Многое из этого документа мы с вами знаем, но, как говорится, повторение - мать учения. Для вашего удобства к этому посту прикреплена PDF версия данного документа.