As a safety mechinism, 4D will create a copy of the database when an action that modifies the structure or datafile is performed. Some example actions are performing a compact, a repair, or a project mode conversion. A copy of the original state of the database is saved before a new copy of the database with the action applied is created. In various case, it can be safe to keep the copy while confirming that the new database is stable, but it is also important to not forget to clean up and delete these copies because if an action like a compact is performed regularly it can begin to saturate the disk space.