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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 

Deploy vm with service offering ‘s’ and memory overcommit factor ‘f’ –

• Static Min memory = Dynamic Min memory = service offering / factor ==  (s/f)
• Static Max memory= Dynamic Max memory =  service offering == s
• Each vm ensured Min memory during contention. •
• No overprovisioning means min=max 
• Min memory = Max memory= service offering == s

Vmware

If vmware.mem.reserve = true 

Reserve memory = (service_offering / memory over provisioning factor)

Else 

Reserve memory = don’t reserve

Same model for cpu.

KVM

TBD

The overcommit ratios are dynamically plugged into the capacity calculations. All the capacity calculations is done based on the overcommitted value of capacities. So if the overcommit ratios is decreased the used capacity may go beyond 100%. 
Example:
Overcommit =2 
capacity = 2GB
capacity after overcommit = 4GB.
Now if we deploy 3 VM of 1 GB each 
used =3GB
free = 1GB
used % = 3/4 *100 = 75%
if the overcommit ratio is decreased to 1
used = 3GB
free = -1GB
used % = 3/2 *100 =150% (will generate alerts based on this.)

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