System and method for determining physical storage space of a deduplicated storage system

Exemplary methods, apparatuses, and systems maintain a plurality of summary data structures corresponding to a plurality of logical file system namespaces representing a plurality of hierarchies of one or more directories having one or more files, each file being stored in the storage system as a plurality of segments in a deduplicated manner. In response to a request to estimate a storage usage by a first of the file system namespace, identify a first of the summary data structures corresponding to the first file system namespace, wherein the first summary data structure stores information summarizing deduplicated segments referenced by one or more files of the first file system namespace. Estimate the storage usage of the first file system namespace based on the first summary data structure and a global summary data structure, wherein the global summary data structure stores information summarizing deduplicated segments referenced by all of the file system namespaces.