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Capacity calculation model will be changed to align with the hypervisors calculation. When a vm is deployed with "x" overprovisioing factor we want to guarantee (service offering of vm / x ) during its lifecycle even though the over provisioning changes.

When the cluster overprovisioing factor = x

• Total Capacity = (actualHardwareCapacity * x)
• Used Capacity = sum (service offering of each running vm) + sum (service offering of each stopped vm in the skipped.counting.hours)

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When the cluster overprovisioing factor is changed to y

• Total Capacity = (actualHardwareCapacity * y)

• Wiki Markup
  Used Capacity = \[sum (service offering of each running vm deployed when factor was x) + sum (service offering of each stopped vm deployed when factor was x in the skipped.counting.hours)\] * y/x \+ 

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• sum (service offering of each running vm deployed when factor was

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• y ) + sum (service offering of each stopped vm deployed when factor

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• was y in the skipped.counting.hours)

Ideally you shouldn't change the over-provisioning factor in a cluster with vms running. This is because the existing vms got deployed with the previous factor. 
Lets say you still want to change the factor. On changing it, both used and total capacity are multiplied by this factor to keep a track of available capacity.

Let's understand the capacity calculation below through an example :-

Cluster – c, 
cpu over provisioning = 1, 
Total cpu = 2GHZ

when we deploy 2VMs of 512Mhz service offering each then 
totalCapacity = 2GHz 
AvailableCapacity = 1GHz
UsedCapacity = 1GHZ

Now change the cpu over provisioning ratio of cluster c to 2
totalCapacity = 4GHz 
AvailableCapacity = 2GHz
UsedCapacity = 2GHZ

Notice the difference in multiplication here. Both used and total capacity are multiplied by this factor. Used Capacity in the new model after changing the factor = (service offering of vm / overcommit it got deployed with) * new overcommit => (1GHZ/1)*2
The reason is want to guarantee minimum cpu in case of contention. So when a vm is deployed with "x" overprovisioing factor we want to gurantee (service offering of vm / x ) during its lifecycle even though the overprovisioning changed.
So the reason we scale the used cpu to keep track of the actual amount of cpu left on the host.

Now if we launch 2 VMs with 1Ghz cpu service offering
totalCapacity = 4GHz 
AvailableCapacity = 0GHz
UsedCapacity = 4GHZ 
Calculation for used capacity for 4vms ((service offering of vm / overcommit it got deployed with) * new overcommit) = 
(512Mhz/1)*2 + (512Mhz/1)*2 + (1Ghz/2)*2 + (1Ghz/2)*2 = 4Ghz

now suppose we change the over provisioning to 3 
totalCapacity = 6 GHz 
AvailableCapacity = 0 GHz
UsedCapacity = 6 GHZ
Calculation for used capacity for 4vms ((service offering of vm / overcommit it got deployed with) * new overcommit) = 
(512Mhz/1)*3 +(512Mhz/1)*3 +(1Ghz/2)*3 + (1Ghz/2)*3 = 6Ghz

Now this is assuming, you haven't stopped and started the vms all this while. Say now you stop and start 1 VM = 512Mhz and another VM = 1Ghz. The over-provisioning factor ratio changes for these vms to 3 each. Note the denominator in the calculation 
totalCapacity = 6 GHz 
AvailableCapacity = 1.5 GHz
UsedCapacity = 4.5 GHZ
Calculation for used capacity for 4vms ((service offering of vm / overcommit it got deployed with) * new overcommit) = 
(512Mhz/3)*3 +(512Mhz/1)*3 +(1Ghz/3)*3 + (1Ghz/2)*3 = 4.5 Ghz

The upside of new model is we are guaranteeing QOS as (service offering of vm / x ) during its lifecycle vs the old model

Hypervisor Calculation

Xenserver 

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