Each allocation model can be used for different levels of performance control and management.

The following table contains information about the suggested use of each allocation model.
Allocation Model Suggested Use
Flex Allocation Model With the flex allocation model, you can achieve a fine-grained performance control at the workload level. By using the flex allocation model, vCloud Director system administrators can manage the elasticity of individual organization VDCs. The flex allocation model uses policy-based management of workloads. With the flex allocation model, cloud providers can have a better control over memory overhead in an organization VDC and can enforce a strict burst capacity use for tenants.
Allocation Pool Allocation Model Use the allocation pool allocation model for long lived, stable workloads, where tenants subscribe to a fixed compute resource consumption and cloud providers can predict and manage the compute resource capacity. Allocation pool allocation model is optimal for workloads with diverse performance requirements. With the allocation pool allocation model, all workloads share the allocated resources from the resource pools of vCenter Server. Regardless if you enable or disable elasticity, tenants receive a limited amount of compute resources. With the allocation pool allocation model, cloud providers enable or disable the elasticity at the system level and the setting applies to all allocation pool organization VDCs. If you use the non-elastic allocation pool allocation, the organization VDC pre-reserves the VDC resource pool and tenants can overcommit vCPUs but cannot overcommit any memory. If you use the elastic pool allocation, the organization VDC does not pre-reserve any compute resources and capacity can span through multiple clusters. Cloud providers manage the overcommitment of physical compute resources and tenants cannot overcommit vCPUs and memory.
Pay-as-You-Go Use the pay-as-you-go model when you do not have to allocate compute resources in vCenter Server upfront. Reservation, limit, and shares are applied on every workload that tenants deploy in the VDC. With the pay-as-you-go allocation model, every workload in the organization VDC receives the same percentage of the configured compute resources reserved. To vCloud Director, the CPU speed of every vCPU for every workload is the same and you can only define the CPU speed at the organization VDC level. From a performance perspective, because you cannot change reservation settings of individual workloads, every workload receives the same preference. Pay-as-you-go allocation model is optimal for tenants that need workloads with different performance requirements to run within the same organization VDC. Because of the elasticity, the pay-as-you-go model is suitable for generic, short lived workloads that are part of autoscaling applications. With pay-as-you-go, tenants can match spikes in compute resources demand within an organization VDC.
Reservation Pool Use the reservation pool allocation model when you need a fine-grained control over the performance of workloads that are running in the organization VDC. From a cloud provider perspective, the reservation pool allocation model requires an upfront allocation of all compute resources in vCenter Server. The reservation pool allocation model is not elastic. The reservation pool allocation model is optimal for workloads that run on hardware that is dedicated to a specific tenant. In such cases, tenant users can manage use and overcommitment of compute resources.