1. A method for incrementally scaling a file system, comprising: adding a new file storage device to a file system having a storage space comprising at least one other file storage device having a plurality of directories and files stored thereon to form a new file system configuration; and migrating a portion of the files from said at least one other file storage device to the new file storage device while hiding such migration from client applications that access files from the file system so as to not affect file access operations requested and performed by the client applications during the migration; wherein the storage space of the file system is partitioned into fragments; the files in the file system are assigned to corresponding fragments; and files are assigned to corresponding fragments based on the directories the files are in.