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Almost all the hosts have the capability to overcommit, and it is up to the admin to make sure of it. Even if the host is not configured properly, cloudstack will try to set the parameters assuming it has capability.

Capacity calculations.

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.)

Alert generation.

All the alerts are generated based on the global threshold values. Will change this behavior once we have
thresholds per cluster.

Upgrade senario.

1.) Disable the overcommit feature in clusters existing before the upgrade. The admin should manually enable it.

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