Files-11, also known as on-disk structure, is the file system used by Hewlett-Packard's OpenVMS operating system, and also (in a simpler form) by the older RSX-11. It is a hierarchical file system, with support for access control lists, record-oriented I/O, remote network access, and file versioning.

Files-11 is similar to, but significantly more advanced than, the filesystems used in previous Digital Equipment Corporation operating systems such as TOPS-20 and RSTS/E. It is also a clear predecessor of NTFS. Many of the concepts used in Files-11 appear in NTFS.

Read more about Files-11:  History, Overview, Directory Layout, Disk Organization and Naming, File Security: Protection and ACLs, Logical Names, Record-oriented I/O: Record Management Services, Physical Layout: The On-Disk Structure, Directories, The Master File Directory, Index File: INDEXF.SYS, Other Files