5.1

Option Description
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.
Reservation Pool
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.
For information on the placement engine and virtual machine shares, rates and limits, see the vCloud
Director User's Guide.
2 Click Next.
Chapter 3 Creating and Provisioning Organizations
VMware, Inc. 35