Virtual MultiVersion File System

The MVFS is one of the building blocks of ClearCase. The functionality it offers is also referenced with the concept of Dynamic Views. It is a protocol, based on NFS, which allows to mount a virtual file system. One may thus access the full databases (optimally multiple reasonably small ones in ClearCase) via the metaphoric interface of a file system.

This may be considered a nice convenience (not necessary for using Revision Control systems, such as e.g. UCM, which mostly use snapshots), but is mandatory for auditing derived object creation.

What is mandatory is the referential transparency needed to identify in a common way (path name) multiple members of a configuration item family.

Now the ClearCase implementation of the MVFS is lacking in several practical ways.

As most of the files used in any software configuration are strictly speaking beyond our control: tools or otherwise part of the installation, products we have downloaded, and extracted from packages; explicitly accessing them from VOBs (the databases), is at least an inconvenience, and often a problem. They may or may not be used from arbitrary locations in the file system. Workarounds exist with ClearCase to preserve and extend the illusion of ubiquity: one can mount VOBs at arbitrary locations (even if it is seldom done), or use symbolic links. Neither goes however far enough. The main problem arises from conflicts with other components of the systems which do not run in a view context (under the SCM).

A second issue is this of preserving the granularity of the VOBs (considered a good thing, for manageability and replication), and its mapping onto the structure of the file system, which obeys other constraints, and is by no means stable.

-- MarcGirod - 26 May 2007