When available resource capacity does not meet the demands of the resource consumers (and virtualization overhead), administrators might need to customize the amount of resources that are allocated to virtual machines or to the resource pools in which they reside.

Use the resource allocation settings (shares, reservation, and limit) to determine the amount of CPU, memory, and storage I/O resources provided for a virtual machine. In particular, administrators have several options for allocating resources.

Reserve the physical resources of the host or cluster.

Ensure that a certain amount of memory for a virtual machine is provided by the physical memory of the ESX/ESXi machine.

Guarantee that a particular virtual machine is always allocated a higher percentage of the physical resources than other virtual machines.

Set an upper bound on the resources that can be allocated to a virtual machine.