This is part of the effort to improve Cinder's Thin provisioning
support. As some operators have been facing problems to determinte
what is the best value for the max_over_subscription_ratio, we
add in this patch a mechanism to automatically calculate this value.
The formula used for calculation is:
max_over_subscription_ratio = 20 if provisioned_capacity_gb == 0 else:
max_over_subscription_ratio = 1 + (provisioned_capacity_gb/(
total_capacity_gb - free_capacity_gb + 1))
Using this formula, the scheduler will allow the creation of a much
bigger number of volumes at the begginning of the pool's life, and
start to restrict the creation as the free space approaces to 0 or
the reserved limit.
Drivers now can set max_over_subscription_ratio = 'auto' and take
benefit of the change. Drivers that somehow use the
max_over_subscription_ratio inside the driver to do any kind of
calculations are incompatible with this new feature and should
get fixed in order to be able to use the feature.
Implements: bp provisioning-improvements
Change-Id: If30bb6276f58532c0f78ac268544a8804008770e
f98c9da944