1. A data storage system accessible by a processor comprising: at least one storage device; at least one computer processor; a hierarchical data storage structure stored in at least one storage device and accessible by the at least one computer processor, the hierarchical data storage structure comprising metadata in a tree structure, each metadata associated with a file or directory; a snapshot identifier associated with a snapshot, of the snapshot comprising members as of a past time, the members comprising a file or directory and descendants of the file or directory, the snapshot identifier stored in at least in a metadata associated with a file or directory; a snapshot tracking data structure comprising the snapshot identifier and configured to store references to metadata associated with snapshot versions of modified members, the modified members comprising members which have been modified or deleted after the past time; and at least one executable software module executed by the at least one computer processor and configured to add a reference to the snapshot tracking data structure, the reference referring to a metadata associated with a snapshot version of a deleted member, the deleted member comprising a member which has been deleted after the past time.