1. A method of operating a client computer to store and retrieve user data files to and from a storage device, comprising: in a file storing operation for a user data file identified by a file identifier: 1) applying a first mapping to the file identifier to originate a corresponding content address in a flat content address space; 2) applying a second mapping to the content address to originate a corresponding multi-level directory name usable to identify a subdirectory in a hierarchical directory structure of a file system of the storage device; and 3) invoking the file system to store the user data file in the subdirectory identified by the multi-level directory name; and in a subsequent file retrieving operation initiated with the file identifier: 4) applying the first mapping to the file identifier to regenerate the content address; 5) applying the second mapping to the regenerated content address to regenerate the multi-level directory name; and 6) invoking the file system using the regenerated multi-level directory name to retrieve the user data file from the subdirectory, wherein the content address is a second content address and the file identifier is a first content address generated by a third mapping from contents of the user data file, and wherein the file storing operation includes applying the third mapping to the contents of the user data file to originate the first content address and returning the first content address to an application for subsequent use by the application in initiating the file retrieving operation.