Comparing Utilization with Allocation

A fundamental but subtle difference exists between allocation and utilization. Allocation refers to how a resource is assigned. In a vSphere environment, allocation refers to how CPU cycles, memory, storage I/O, and network bandwidth are distributed to a virtual machine (VM) or group of VMs. Utilization is how those resources are used after they are allocated to the VMs from the host or hosts.

vSphere provides three mechanisms for allocation: reservations (guaranteed allocations of resources), limits (upper bound on the allocation of resources), and shares (prioritized access to resource allocation during periods of resource contention). Although these mechanisms are extraordinarily powerful and useful—as you saw in Chapter 11, “Managing Resource Allocation”—they do have their limits.

How do we address situations where a resource is highly utilized on one host and lightly utilized on another? None of the three mechanisms...