1. A data storage system accessible by a processor comprising: a hierarchical data storage structure with directory nodes and file nodes in a tree structure, at least some of said directory nodes having a plurality of file nodes as children; a snapshot identifier associated with a directory to designate a snapshot as of a given time, the snapshot including the directory and all subdirectories, if any, and all files under the directory; and the same snapshot identifier associated with each of the subdirectories, if any, and files under the directory.