• Preetam Zare

Pay-as-you-go Resource Allocation Model behind the scene

When organization is created it won’t have any resources. We must assign resources to the organization. When you allocate resource to organization you end up creating organization vDC. These resources comes from Provider vDC as explained in earlier blog. We need resource allocation model to ensure right kind of service could be delivered to the consumers/BU. Only CPU and Memory are part of resource allocation model.

vCloud provides three different types of resource allocation model


Allocation Pool

Reservation Pool

Pay-as-you-go model is primarily used for varying workload and it is most suitable for developer and QA workload. This model is unique in many ways. In this model resources are applied per VM basis. It means memory and CPU limits and reservations are applied per VM basis and not at resource pool level. Resources are allocated only when workload are powered ON. This model assumes there is infinite pool of resource. As cloud administrator you have to ensure this assumption is not taken seriously by organization administrator. You can control it using various options. These are CPU, Memory quota and maximum number of VMs that can be started. You can use either of parameter to control resource usage. Over and above there is way you can also provide some kind of guarantee (reservation) on CPU and Memory when VM is powered ON.

In below screen organization vDC is created with the following values

CPU quota = 6 GHz (read this as a limit)

CPU Resources Guaranteed = 50%

vCPU Speed = 1 GHz (read this as a limit per vCPU at VM level)

Memory quota = 3.33 GB ((read this as a limit))

Memory Resources Guaranteed=50%

lets create three VM’s each with 2 vCPU and 512 GB memory. Below table explains what will be VM level memory and limit configuration

From the above table it can be read that Total CPU configured per VM will be 2000 GHz (see below –it is configured as limit) and 50% of total resources are guaranteed (see below out 2000 MHZ, 1000 MHZ are reserved).

One thing you must note 6 GHZ and 3.33 GB RAM is cumulative limit. It means you can have 1 VM of 6 GHZ and 3.33 GB RAM or you can have any number of VMs as long 6 GHZ and 3.33 GB RAM resources are not exceeded.

To prove it, I have configured two VM with 3 vCPU which amounts to 6 GHZ. I Left the 3rd VM un touched to 2 vCPU. Since we have allocated only 6 GHZ it cannot powered on 3rd VM.

Similar fact can be proved for memory reservation as well


Quota you configure in this allocation model is the maximum a VM or VM’s in this Organization vDC can use. CPU and Memory quota is the another way to limit the resource allocated in Pay-as-you-go model.If you do not configure any quota then there is high risk that provider vDC’s entire resource might get consumed without being warned. So please use these controls always.

Reservations and limits are set at the VM level and not at the resource pool level.

Another point one should note, it is not the consumption of resources by VM but allocation of resource to the VM are considered by admission control before powering on additional VM. E.g :- If you allocate 6 GHZ to this organization vDC and you configure 1 VM with 6 vCPU (assuming vCPU speed =1 GHz), you cannot power ON another VM as 6 GHZ limit is reached. However it not concerned about how much out of 6 GHz this VM is actually using. This is monitoring consumer must do and make a decision to resize it. Of course provider can provide these utilization numbers if requested.

If you look at it, it is different kind of admission control compared with the one we have learnt from vSphere HA cluster. Therefore design Pay-as-go model accordingly.

In next blog post I will be discussing the allocation pool model.

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