Chapter 20: The Files-11 File System

Overview

A file system is that part of an operating system that controls the storage and manipulation of files on media, such as disks. At first thought, that may seem like a rather straightforward task. However, to carry out this function, a file system supporting a multiuser operating system must perform a variety of difficult jobs.

A successful file system must do the following:

• Impose upon a blank medium a structure capable of representing highly organized data (The structure must usually be hierarchical, that is, to support files in directories and subdirectories)

• Be able to multiplex the use of storage units among many concurrent accessors

• Contain internal synchronization for all accesses

• Enforce security by allowing data access only to those who have legitimate authority

• Manage the sharing of individual files by multiple accessors concurrently

• Isolate faults stemming from imperfect physical media or improper access (This implies minimizing the potential for data loss due to hardware faults.)

• Provide a standard set of interface routines to upper layers of the operating system and user programs and make them apply equally to devices with dissimilar interfaces.

Finally, a file system must do all these things with minimal impact on system performance.

The file system supplied with OpenVMS is Files-11. Within the Files-11 design, there are two main specifications for the physical arrangement of data on a disk. They are ODS-2 (for On-Disk Structure level 2, used for most of the history of OpenVMS) and ODS-5 (a newer disk structure that has been gaining popularity for the past few years).

Note 

The Files-11 file system is concerned with files, directories, and free space on a disk. It is not to be confused with OpenVMS Record Management Services (RMS), which is concerned with the data format within files.

Most of the information presented here applies to both structure levels, and exceptions are noted. For the general user, the most important practical difference is a more flexible set of file-naming rules introduced with ODS-5. Examples in this book conform to ODS-2 rules.