Appendix C: Capacity Planning : vCloud Administrator (Service Provider) Perspective : Pay As You Go Model
Pay As You Go Model
When an organization virtual datacenter is created in the Pay As You Go model, a resource pool is instantiated with expandable reservations. As such, the customer organization virtual datacenters contained on that provider virtual datacenter can grow to consume all of the available provider virtual datacenter resources. While this could be true in any vSphere environment, the added challenge in a vCloud is the use of reservations at the vApp level. When an organization virtual datacenter is created out of a provider virtual datacenter using the Pay As You Go consumption model, a %guarantee is configured for CPU and memory. This is applied to each vApp or virtual machine within a vApp. For example, if the service provider configures the organization virtual datacenter with a 50% guarantee for CPU and 75% guarantee for memory, then the customer creates a virtual machine consuming 1 vCPU of 1GHz and 1GB of memory, a reservation for that virtual machine is set at 50% of 1GHz, or 0.5 GHz and 75% of 1GB, or 0.75GB of memory.
Because there is no way of knowing how a customer may define virtual machine templates in private customer catalogs—and because an organization’s virtual datacenters can expand on demand, VMware recommends the following:
*Calculate the total available CPU and memory resources (less an amount reserved for global catalog templates), adjusted by the cluster redundancy ratio, at the provider virtual datacenter level.
*Establish a CPU and memory %RESERVED threshold at the provider virtual datacenter level.
*Establish the %RESERVED for the provider virtual datacenter at a number in the 60% range initially.
*As the total amount of reserved CPU or reserved memory approaches the %RESERVED threshold, do not deploy new organization virtual datacenters in that provider virtual datacenter without adding additional resources. If the corresponding vSphere cluster has reached its maximum point of expansion, a new provider virtual datacenter should be deployed and any new organization virtual datacenter should be assigned to the new provider virtual datacenter. In this way there is a 40% of expansion capacity for the existing organization virtual datacenters in the case where the provider virtual datacenter has reached its maximum point of expansion.
*CPU and memory overcommitment can be applied, and if so, the %RESERVED value should be set lower than if no over-commitment is applied due to the unpredictability of the virtual machine sizes being deployed (and hence reservations being established).
*Monitor the %RESERVED on a regular basis and adjust the value according to historical usage as well as project demand.