For some reasons when we delete a MergeTree Part in atomic way something may go wrong - and leave the delete_tmp_* folder or it's part on disk. This may lead to a disk space consumption over time.

It's suggested to delete this folders at least at server start-up. Current solution with clearOldTemporaryDirectories() doesn't work because this method is called periodically from a thread without any locks or advisory on folders - it may cause the race conditions.