What is a vCPU?
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It is not an allocated model it is a concurrent consumed model or a high water mark. This means that customers can utilise very large amounts of overprovisioning if so desired but this might mean that in peak times when many of your VMs are running high CPU workloads you will be limited in terms of available CPU resources. In the event of this the VMs will go into a CPU Wait state for available resources when other VMs in your own environment are not using them.
Scenarios
- Customer has 100 VMs all with 2 vCPUs assigned. The customer is paying for
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- 200GHz. In this scenario all VMs will be able to utilise all the resources that have been paid for as even if every VM is running both CPUs at 100% the total will only be
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- 200GHz which is what has been assigned to the customers Virtual Data Centre. There is no conceivable event where any VM does not have access to its resources.
- Customer has 8 VMs all with 1 vCPU assigned, 6 powered on and 2 powered off. The customer is paying for
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- 12GHz. In this scenario the 6 powered on VM are working fine performance wise. The customer has powered on an additional 2 VMs and now their users are complaining of performance issues. With the 8 VMs powered on the requirements could burst up to
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- 16GHz but the customer is only paying for
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- 12GHz so the customer would need to either be aware that performance may be an issue, power off the additional 2 VMs or increase the allocated resources in the environment to
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- 16GHz.
- Customer has 100 VMs all with 1 vCPUs assigned. The customer is paying for 30GHz. In this scenario the customer is obviously drastically under allocating CPU resources for their VDC as the provisioned amount would be 250GHz. In day to day usage of the VMs they are generally only 10% utilised in terms of CPU. 10% of a single vCPU is .2GHz and for the 100 VMs this is 20GHz in total day to day. In this scenario whilst the customer is not paying for nearly enough CPU it is not constrained in normal activities as they are still under the 30GHz total that has been allocated. A warning in this scenario though, if any VMs start doing work more than the average then there may be periods where the total GHz used is over the 30GHz so it will be resource constrained when this occurs and potential for users to create service desk tickets complaining of performance problems.
- Customer has 100VMs with a mixture of 1-4 vCPUs assigned. The customer has allocated 200 GHz to the Virtual Data Centre. Generally in the environment the VMs day to day run at 30% which would have a total resource requirement in GHz terms of 260GHz. In this scenario the VMs will constantly be resource constrained by our systems as there is not enough resources provided so it is expected that the VMs will run slower than they should be in normal circumstances as they are continually in a CPU Wait state.
So how much should I buy?
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