[comp.os.vms] comments on disk structure

MCGUIRE@GRIN1.BITNET ("The Sysco Kid ", McGuire,Ed) (04/14/88)

Actually, Digital has made significant changes to disk structures in the
past, both in VMS and in RSTS/E.  This precedent would make me very reluctant
to write any code that depends upon the internal structure of a directory
file.

In VMS they added support for longer filenames, access control lists, and so
forth.  I don't normally program at that level on VMS but I'd imagine that the
changes in data structures required for this support could break programs that
make assumptions about their layout and size.  This could apply both to
the directory structure and also to the retrieval and security database
(INDEXF.SYS).

In RSTS/E they added `large file' support to a structure which originally
limited the size of a file to 32767 or 65535 due to the size of a 16-bit word.
Anyone who had written code to look at the directory structure and retrieval
pointers had to rewrite it or it would die when it encountered a large file.

Later, they redesigned the directory structure, removing retrieval pointers
from the master file directory and creating a new hierarchical retrieval
structure.  This change broke any program that took advantage of knowledge of
the structure of directory or retrieval blocks.

Any documentation that Digital provides on the layout of those data structures
is sketchy.  This has always encouraged folks to use system services to
manipulate the disk structure instead.  Digital appears to make the effort to
maintain compatibility of system services from release to release.

Conclusion: Digital does make changes to disk directory and retrieval data
structures. Programs that call system services are much less likely to break
later than programs that assume a particular internal implementation of disk
structure.

Ed