Today, the `update_dirs` tool will allow the removal of a directory, even if tablet data exists on it. For tablet servers, this means upon the next startup, those tablets will be re-replicated to other servers. For masters, this means the node will crash.
While a well-provisioned Kudu cluster should be able to handle both of these cases, it would be nice to redistribute the tablet's data locally to avoid either outcome. This entails moving the data blocks from the removed directory, and rewriting the tablet metadata (which keeps a record of the data directories across which the tablet's data is stored) to exclude the removed directory.