5.5

Understanding Allocation Models
An allocation model determines how and when the provider virtual datacenter compute and memory
resources that you allocate are committed to the organization virtual datacenter.
Allocation Pool Allocation Model
With the allocation pool allocation model, a percentage of the resources you allocate from the provider
virtual datacenter are committed to the organization virtual datacenter. 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 5.1.2, system administrators can configure allocation-pool organization
virtual datacenters to be elastic or non-elastic. This is a global setting that affects all allocation-pool
organization virtual datacenters. See “Modify General System Settings,” on page 129.
By default, allocation-pool organization virtual datacenters have a elastic allocation pool enabled. Systems
upgraded from vCloud Director 5.1 that have allocation-pool organization virtual datacenters with virtual
machines spanning multiple resource pools have elastic allocation pool enabled by default.
When allocation-pool virtual datacenters have the elastic allocation pool feature enabled, the organization
virtual datacenter spans and uses all resource pools associated with its provider virtual datacenter. 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 virtual datacenter without CPU being a bottleneck factor.
When a virtual machine is created, the placement engine places it on a provider virtual datacenter resource
pool that best fits the requirements of the virtual machine. A subresource pool is created for this
organization virtual datacenter under the provider virtual datacenter resource pool, and the virtual machine
is placed under that subresource pool.
When the virtual machine powers on, the placement engine checks the provider virtual datacenter resource
pool to ensure that it still can power on the virtual machine. If not, the placement engine moves the virtual
machine to a provider virtual datacenter resource pool with sufficient resources to run the virtual machine.
A subresource pool for the organization virtual datacenter is created if one does not already exist.
The subresource pool is configured with sufficient resources to run the new virtual machine. The
subresource 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 virtual datacenter. The subresource pool's CPU limit is increased by
the number of vCPUs that the virtual machine is configured with times the vCPU frequency specified at the
organization virtual datacenter level. The CPU reservation is increased by the number of vCPU configured
for the virtual machine times the vCPU specified at the organization virtual datacenter level times the
percentage guarantee factor for CPU set at the organization virtual datacenter level. The virtual machine is
reconfigured to set its memory and CPU reservation to zero and the virtual machine placement engine
places the virtual machine on a provider virtual datacenter resource pool.
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 subresource pool. This model can take advantage of new resources
added to the provider virtual datacenter.
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 change might
involve a minor cost to move the virtual machine disk files to a new resource pool.
When the elastic allocation pool feature is disabled, the behavior of allocation-pool organization virtual
datacenters is similar to the allocation pool model in vCloud Director 1.5. In this model, the vCPU frequency
is not configurable. Overcommitment is controlled by setting the percentage of resources guaranteed.
vCloud Director Administrator's Guide
26 VMware, Inc.