Have you been affected by the Allocation Pool changes in VCD 5.1 when you upgraded?
If you have been, and are looking for a way to make this allocation model the same as in VCD 1.5, VMware has included a feature in VCD 5.1.2 that allows you to change it back to the original way it was conceived.
First lets look at the changes in the Allocation Pool.
Massimo ReFerre has written a great article showing the differences between the two versions of the allocation model http://it20.info/2012/10/vcloud-director-5-1-1-changes-in-resource-entitlements/
In this article he provides a comparison chart for you to easily see the differences, and gives a nice review of allocation models at the end of the article.
OK but what does all this mean for me?
As I am sure you are more confused than when you started reading this post… perhaps it makes sense to put a stake in the ground and underline advantages and disadvantages of the three models with vCloud Director 5.1.
The PAYG model is the most simplistic of the three. This model allows the tenant to scale without pre-configured limits. It does also allow cloud consumers to scale without any contractual agreement on resources. Sophisticated capping mechanisms now allows the cloud administrator to limit a tenant based on number of VMs, CPU and memory resources. One thing to notice is that all VMs in a PAYG are standalone entities that have specific limits and guarantees that can’t be shared with the other VMs in the same tenant. So if a VM is not using all the reserved capacity available to it, that capacity cannot be used by other VMs in the tenant that are demanding more resources. The other typical disadvantage of this model is that it’s based on a first-come-first-served basis. Given the cloud consumer didn’t subscribe to allocated or reserved resources, the system may refuse to deploy VMs at any time depending on the status of resource consumption on the Provider vDC.
The Allocation Pool model is interesting because it allows the cloud administrator (but not the cloud consumer) to oversubscribe resources. The level of oversubscription is set by the cloud administrator at the time the Org vDC is created and the cloud consumer cannot alter those values. The most evident advantage of this model is that the cloud consumer has a set of allocated and reserved resources that has been subscribed (typically for a month). The other advantage of this model is that all VMs in the same Org vDC can share CPU and memory resources inside a bucket of resources that is dedicated (yet oversubscribed) to the tenant. The disadvantage of this model is that the cloud consumer can deploy a finite number of VMs before their total resources hits the limit of the Org vDC.
The Reservation Pool model is radically different from the above two. In this model a Resource Pool is completely dedicated and committed to the cloud consumer. This means that all oversubscription mechanisms are delegated to the tenant thus giving to the cloud consumer the flexibility to choose the oversubscription ratio of resources. The disadvantage of this model is that the cloud administrator cannot benefit from oversubscribing resources at Org vDC instantiation, given the allocated resources to the tenants are 100% reserved. This means that the cloud consumer will have to absorb the cost of this premium service from the cloud provider. Note that the Reservation Pool model (with vCloud Director 5.1) is the only one that doesn’t support elasticity thus further limiting the cloud provider flexibility and architectural choices.
I would really recommend reading the whole article, it is really informative.
But what happens if you dont want to use the new type of allocation model?
Perhaps you are upgrading from VCD 1.5, and your whole business model is based on this version of the allocation model? Well now there is a way to keep the Allocation Pool model the same as VCD 1.5.
To do this we need to disable the option “Make Allocation pool Org VDCs Elastic”. We do this by following the steps below:
- Login to the Cloud as a System Administrator
- Select Administration
- Select General
- Scroll to the bottom
- You will see a section titled Miscellaneous
- You will see a Check Box “Make Allocation pool Org VDCs Elastic”
- Remove the tick from the checkbox and click Apply
- If you are watching vCenter you will see a number of tasks run changing the resource pool settings.
I would expect that when you do an in place upgrade from VCD 1.5 to VCD 5.1.2 it would automatically disable this checkbox and keep the Allocation Model the same, however I have not tested this yet. I will update the article when I have the opportunity to test.
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