A system and method for quickly and efficiently determining the number of overwritten blocks between two data containers served by a storage system is provided. A novel vbnspace namespace is provided that is "parallel" to the file system (the "active" file system) and tracks location information (i.e., indirect block pointers) of level 0 data blocks within each data container, e.g., the active file system or a PCPI. The vbnspace namespace effectively eliminates the conventional level 0 data blocks from a storage viewpoint to thereby reduce the quantity of information needed to determine the number of overwritten blocks between the data containers.