lddriver technical journal jun2005

LDdriver is an OpenVMS device driver for creating and managing virtual disks, developed in 1985 and integrated into OpenVMS since V7.3-1. It offers three primary modes of operation:

1. FILE mode: Uses an arbitrary file on a physical disk as a virtual disk.
2. LBN (Logical Block Number) mode: Partitions a physical disk into logical block ranges, offering efficient disk utilization and the ability to map foreign volumes.
3. REPLACE mode: Provides direct LDdriver access to a physical device, primarily for its advanced debugging features.

The driver supports sharing virtual disks across cluster nodes (with strict consistency rules) and offers powerful diagnostic capabilities: real-time I/O Tracing to monitor all disk activity, and Watchpoints that can trigger actions (Error, Suspend, OPCOM message, or System Crash) when specific logical blocks are accessed, invaluable for debugging and simulating disk conditions. LDdriver's operations are managed via DCL commands or a callable interface, with cluster-wide coordination handled by the distributed lock manager. It is available as freeware and often includes source code.