VAX-11 Disk Save and Compress

This document serves as a user's guide for the VAX-11 Disk Save and Compress (DSC) utilities (DSC1 and DSC2), released in August 1978. It details the capabilities and usage of these tools, which are designed to copy Files-11 disk files to either disk or magnetic tape, and to restore them from tape back to disk.

A primary function of DSC is to optimize disk storage by reallocating and consolidating data. It concatenates disk files and their extensions into contiguous data blocks, reducing the number of retrieval pointers and file headers required, which improves access efficiency. If data is transferred to a larger disk, the new disk maintains the original attributes while making the additional space available. DSC can also aid in recovering data from disks with unreadable sectors by copying garbled content and flagging it for correction.

The document distinguishes between two main data transfer operations:

• Disk-to-Disk Copy: A direct, one-step process where data is read from one disk and written to another, with the new disk files being immediately accessible.
• Disk-to-Tape Copy: A two-step process (disk-to-tape, then tape-to-disk). Data stored on tape by DSC is in a special format, making it unusable until it's copied back to disk. This method is used for permanent storage or reconstruction. DSC only supports tapes it has previously created.

DSC supports two Files-11 structure levels: DSC1 handles Structure Level 1 disks (transportable across VAX/VMS, RSX-11M, RSX-11D, and IAS systems), while DSC2 handles Structure Level 2 disks (exclusive to VAX/VMS). A self-contained, stand-alone version of DSC2 is also available for single-disk system backups/restores and creating system disks from tape.

The guide describes the DSC command format, including device specifications, file labels, and various qualifiers like /REWIND (to reposition tapes), /BAD (for bad block management), /APPEND (to add files to existing tape sets), /NONMERGE (to copy files without consolidating extensions), /DENSITY (to set tape density), /COMPARE (to compare devices), and /VERIFY (to copy and then compare). Data transfer can occur in 'merge mode' (default), which consolidates file extensions and optimizes block allocation, or 'nonmerge mode', which preserves the original file number order without full consolidation.

Finally, the document includes a comprehensive list of DSC-generated messages, categorized as warnings or fatal errors, providing text, explanations, and corrective actions for various operational issues. The guide is intended for users with VAX-11/780 experience and familiarity with VAX/VMS operating system concepts.