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Look into the option:

disk_allocation_ratio=1.0 (FloatOpt) virtual disk to physical disk allocation ratio

Then you can look at nova scheduler to see how it calculates where to put new instances.

Basically you would want to configure the disk allocation ration to be 1:1 if you are worried about hitting the limit, or even leave some space in reserve. You could also achieve this with:

reserved_host_disk_mb =

The disk uses/available gets reported back to the scheduler from nova-compute running on the compute nodes. It updates what resources it still has available so that the scheduler can figure out where to put the next instance that gets launched. Depending on your deployment the calculation could be different, if you customize then you can add other attributes to look for when launching instances.