Overview
"Files-11" is the general term for five separate filesystems, known as on-disk structure (ODS) levels 1 through 5.
ODS-1 is the flat filesystem used by the RSX-11 OS, supported by older VMS systems for RSX compatibility, but never used to support VMS itself; it has been largely superseded by ODS-2 and ODS-5.
ODS-2 is the standard VMS filesystem, and remains the most common filesystem for system disks (the disk on which the operating system is installed).
Although seldom referred to by their ODS level designations, ODS-3 and ODS-4 are the Files-11 support for the CD-ROM ISO 9660 and High Sierra Format filesystems, respectively.
ODS-5 is an extended version of ODS-2 available on Alpha and IA-64 platforms which adds support for case-preserving filenames with non-ASCII characters and improvements to the hierarchical directory support. It was originally intended for file serving to Microsoft Windows or other non-VMS systems as part of the "NT affinity" project, but is also used on user disks and Internet servers.
Read more about this topic: Files-11