[large-scale][cinder] backend_native_threads_pool_size option with rbd backend

Hey all, Is there any recommendation on the number of threads to use when using RBD backend (option backend_native_threads_pool_size)? The doc is saying that 20 is the default but it should be increased, specially for the RBD driver, but up to which value? Is there anyone tuning this parameter in their openstack deployments? If yes, maybe we can add some recommendations on openstack large-scale doc about it? Cheers, Arnaud.