DTJ Volume 8 Number 2 1996 - Digital Technical Journals

Providing a Compatible File System Interface
Application compatibility can be described in two
ways : compatibility at the file system interface and
compatibility of the on-disk structure. Since only specialized
applications use knowledge of the on-disk
structure and maintaining compatibility at the interface
level is a teature of the Open VMS system, the
Spiralog tile system preserves compatibility at the file
system interface level only. In the section Files-1 1 and
the Spiralog File System On-disk Structures, we give
an overview of the major on-disk diffe rences bet\veen
the t\VO file systems.
The level of interface compatibility wou ld have a
large impact on how well users adopted the Spiralog
file system. If data and applications could be moved to
a Spiralog volume and run unchanged, the file system
would be better accepted . The goal tor the Spiralog
file system was to achieve 100 percent interface compatibi
lity for the majority of existing applications. The
implementation of a log-structured ti le system, however,
meant that certain features and operations of the
Files-1 1 file system could not be supported.
The Open VMS operating system provides a number
of ti le system interfaces that are called by applications.
This section describes how we chose the most compatible
tile system interface. The OpenVMS operating
system directly supports a system-level call interface
(QIO) to the ti le system, which is an extremely complex
interface.' The QIO interface is very specific to
the OpenVMS system and is difficult to map directly
onto a modern ti le system intedace. This interface is
used infrequently by applications but is used extensively
by Open VMS utilities.
Open VMS File System Environment
This section gives an overview of the general
OpenVMS file system environment, and the existing
Figure 1
r-
n 1 HIGH-LEVEL
APPLICATIONS
Open VMS and the new Spiralog file system intertaces.
To emulate the Files-1 1 file system, it was important to
understand the way it is used by applications in the
OpenVMS environment. A brief description of the
Files-1 1 and the Spiralog file system interfaces gives an
indication of the problems in mapping one interface
onto the other. These problems are discussed later in
the section Compatibility Problems.
In the Open VMS environment, applications interact
with the ti le system through various interfaces,
ranging from high-level language interfaces to direct
ti le system calls. Figure 1 shows the organization of
interfaces within the Open VMS environment, including
both tl1e Spira log and the Files-1 1 file systems.
The fo llowing brietly describes the levels of interface
to the tile system.
• High-level language (HLL) libraries. H LL libraries
provide tile system fu nctions tor high-level
languages such as the Standard C library and
FORTRAN I/0 fu nctions.
• OpenVMS language-specific libraries. These
libraries offe r OpenVMS-spccific ti le system fu nctions
at a high level. For ex;mple, lib$create_dir( )
creates a new directory with specitic OpenVMS
security attributes such as ownership.
• Record Management Services. The OpenVMS
Record Management Services (RMS) are a set of
complex routines that fo rm part of the Open VMS
kernel. These routines