Only a percentage of the resources you allocate from the provider vDC are committed to the organization vDC. You can specify the percentage for both CPU and memory. This percentage is known as the percentage guarantee factor, and it allows you to overcommit resources.

Starting with vCloud Director 5.1, Allocation Pool organization vDCs are elastic by default. This means that the organization vDC spans and utilizes all resource pools associated with its provider vDC. As a result, vCPU frequency is now a mandatory parameter for an Allocation Pool.

Set the vCPU frequency and percentage guarantee factor in such a way that a sufficient number of virtual machines can be deployed on the organization vDC without CPU being a bottleneck factor.

When a virtual machine is created, the placement engine places it on a provider vDC resource pool that best fits the requirements of the virtual machine. A sub-resource pool is created for this organization vDC under the provider vDC resource pool, and the virtual machine is placed under that sub-resource pool.

When the virtual machine powers on, the placement engine checks the provider vDC resource pool to ensure it still has the capacity to power on the virtual machine. If not, the placement engine moves the virtual machine to a provider vDC resource pool with sufficient resources to run the virtual machine. A sub-resource pool for the organization vDC is created if one does not already exist.

The sub-resource pool is configured with sufficient resources to run the new virtual machine. The sub-resource pool's memory limit is increased by the virtual machine's configured memory size, and its memory reservation is increased by the virtual machine's configured memory size times the percentage guarantee factor for the organization vDC. The sub-resource pool's CPU limit is increased by the number of vCPU the virtual machine is configured with times the vCPU frequency specified at the organization vDC level, and the CPU reservation is increased by the number of vCPU configured for the virtual machine times the vCPU specified at the organization vDC level times the percentage guarantee factor for CPU set at the organization vDC level. The virtual machine is reconfigured to set its memory and CPU reservation to zero and placed.

The benefits of the Allocation Pool model are that a virtual machine can take advantage of the resources of an idle virtual machine on the same sub-resource pool and that this model can take advantage of new resources added to the provider vDC.

In rare cases, a virtual machine is switched from the resource pool it was assigned at creation to a different resource pool at power on because of a lack of resources on the original resource pool. This might involve a minor cost to move the virtual machine disk files to a new resource pool.

Resources are only committed when users create vApps in the organization vDC. You can specify a percentage of resources to guarantee, which allows you to overcommit resources. You can make a Pay-As-You-Go organization vDC elastic by adding multiple resource pools to its provider vDC.

Resources committed to the organization are applied at the virtual machine level.

When a virtual machine is powered on, the placement engine checks the resource pool and assigns it to another resource pool if the original resource pool cannot accommodate the virtual machine. If there is no sub-resource pool for the resource pool, vCloud Director creates one with an infinite limit and zero rate. The virtual machine's rate is set to its limit times its committed resources and the virtual machine is placed.

The benefit of the Pay-As-You-Go model is that it can take advantage of new resources added to the provider vDC.

In rare cases, a virtual machine is switched from the resource pool it was assigned at creation to a different resource pool at power on due to a lack of resources on the original resource pool. This might involve a minor cost to move the virtual machine disk files to a new resource pool.

In the Pay-As-You-Go model, no resources are reserved ahead of time, so a virtual machine might fail to power on if there aren't enough resources. Virtual machines operating under this model are also unable to take advantage of the resources of idle virtual machines on the same sub-resource pool, since resources are set at the virtual machine level.

All of the resources you allocate are immediately committed to the organization vDC. Users in the organization can control overcommitment by specifying reservation, limit, and priority settings for individual virtual machines.

Because there is only one resource pool and one sub-resource pool in this model, the placement engine does not reassign a virtual machine's resource pool when it is powered on. The virtual machine's rate and limit are not modified.

With the Reservation Pool model, sources are always available when needed. This model also offers very fine control over virtual machine rate, limit, and shares, which can lead to optimal usage of the reserved resources if you plan carefully.

In this model, reservation is always done at the primary cluster. If there are not sufficient resources to create an organization vDC on the primary cluster, the organization vDC creation fails.

Other limitations of this model are that it is not elastic and organization users might set non-optimal shares, rates, and limits on virtual machines, leading to underutilization of resources.