VMS File System Internals

This document, "VMS File System Internals" by Kirby McCoy, published in 1990 by Digital Press, provides an in-depth look into the internal components of the VMS Version 5.2 file system.

The book is structured into eight chapters, covering:

1. Introduction to the VMS File System: Traces the evolution of the VMS file system, including the creation of the Extended QIO Processor (XQP) to handle file I/O in a VAXcluster environment, and outlines the user interface through the VMS I/O system and Queue I/O Request Service ($QIO).
2. Files-11 On-Disk Structure: Details the basic concepts of volumes, files, and directories, focusing on the Files-11 On-Disk Structure Level 2 (ODS-2). It describes volume identification, integrity, volume sets (tightly and loosely coupled), file identification (File ID, file header, retrieval pointers), directory structures, and various reserved files (Index File, Storage Bitmap File, Bad Block File, Master File Directory, Core Image File, Volume Set List File, Continuation File, Backup Journal File, Pending Bad Block Log File).
3. Volume Structure Processing: Explains the processes of initializing, mounting, and dismounting a volume, and the associated I/O database structures like Volume Control Blocks (VCB), Window Control Blocks (WCB), ACP Queue Blocks (AQB), File Control Blocks (FCB), and Relative Volume Tables (RVT). It also covers rebuilding bitmap and disk quota files.
4. Cache Processing on a Single Node: Discusses the fundamentals of I/O buffer caching, including cache header, buffer and lock block descriptors, LBN and Lock Basis Hash Tables, buffer pools (storage bitmap, directory data, file header, directory index), and specialized caches (extent, file ID, quota). It also covers buffer management, disk read/write operations, and system-wide buffer validation.
5. The ACP Functions: Describes the major and miscellaneous functions of the Ancillary Control Process (ACP), such as access, create, delete, modify, deaccess, and ACP control. It explains the ACP-QIO interface, disk quota operations, directory manipulation, space management, attribute handling, dynamic highwater marking, spool file processing, access control list processing, dynamic bad block processing, and window handling.
6. The XQP and I/O Processing: Provides a detailed explanation of the XQP's role in I/O processing, including its initialization, call interface (I/O Request Packet, Function Decision Table, Driver Dispatch Table), internal dispatching, code execution, stack switching, transaction stalling, error handling, and termination procedures.
7. Serialization of File System Activity: Focuses on synchronization techniques, including the Distributed Lock Manager, spin locks, mutexes, XQP-internal locks, and system-owned locks. It details serialization of access to files, volumes, and shared data structures (FCBs, VCBs, caches), and addresses deadlock considerations and file system lock indexes.
8. File System Operation in a VAXcluster Environment: Explores file system operations within a VAXcluster, covering cluster-wide disk mounting, various locking mechanisms (volume allocation, arbitration, cache flush, quota cache, blocking locks), access arbitration, system blocking routines (for volume activity, dynamic quota cache entry lock passing, FCB invalidation, cache flushing), cache processing (lock value blocks, cache invalidation), and user invalidation of cached buffers.

The book is intended for software specialists, system programmers, and users familiar with VAX architecture and VMS, aiming to provide a deep understanding of the VMS file system's internal workings, data structures, and synchronization mechanisms.