An administrator can customize the amount of resources allocated to a virtual machine, or to the resource pool in which those virtual machines reside, by modifying resource controls. Each of the primary four resources (CPU, memory, network, and storage) can be controlled, but network and storage require the use of the advanced features of Network I/O Control and Storage I/O Control. There are three resource controls that are available to determine how resources are provided to a virtual machine; these are shares, limits, and reservations. When an ESXi host's memory or CPU resources are overcommitted, a virtual machine's allocation target is somewhere between its specified reservation and specified limit, depending on the VM's share and the system load. This is something that we will explore later in this chapter.