This overprovisioned cap coverage % metric helps to monitor how much capacity is available to cover the overprovisioned virtual capacity of the thin provisioned (space efficient) VDisks.
This helps the administrator to avoid to run into out of storage situation.
What does Overallocation coverage mean?
As you can see within the table tooltip for the new Coverage % metric, this is the percentage value of "how much unused overprovisioned virtual capacity is covered with free capacity in the providing pools":
Pict 2: Leave the mouse above the column header and you will receive this information
A thin provisioned VDisk (TPV) or compressed VDisk uses less space in the backend in comparison to what the server/host can see as available Storage.
(eg. 1 TiB virtual capacity visible to the server/host vs. 250 GiB real capacity allocated in the backend layer (pool). So this VDisk can grow up to 1 TiB. In this case it could need ~750 GiB in addition in the pool)
The question is: "Do you have this capacity needed available in your pool? Or how much of the possible needed capacity are you covering with your current pool configuration?"
These questions can now be answered very easy and very exactly with the new Overallocation coverage % metric.
Pict 3: Shows how "overprovisioned cap coverage %" is calculated