Authorizes use of file. Separation of directory service from file server allows a variety of directory service types. Directory files stored by the file service. Read Write Execute 1. Client request includes file name, user id, requested modes of access.
Directory service checks users' authorization.
B-trees used in XDFS for translation of UFIDs to file locations. Page blocks are allocated as needed. Who opened which files. Where to read from next in file. File server reboots.