Due to the way RADOS works, RBDs are thin provisioned;  that is to say, the underlying objects are only provisioned once data is written to the logical block address that corresponds to that object. There are no safeguards around this; Ceph will quite happily let you provision a 1 PB block device on a 1 TB disk, and, as long as you never place more than 1 TB of data on it, everything will work as expected. If used correctly, thin provisioning can greatly increase the usable capacity of a Ceph cluster as VMs, which are typically one of the main use cases for RBDs, likely have a large amount of whitespace contained within them. However care should be taken to monitor the growth of data on the Ceph cluster; if the underlying usable capacity is filled, the Ceph cluster will effectively go offline until space is freed.