From: Bill Todd [billtodd@foo.mv.com]
Sent: Sunday, July 11, 1999 2:01 PM
To: Info-VAX@Mvb.Saic.Com
Subject: Re: Whither VMS?

Damn - knew I'd forget something significant.  ODS-2 has its roots (as do
most - all? - file systems of similar vintage) in the concept that a file
system is confined to a single, and essentially homogeneous, volume.  ODS-2
was advanced for its time it that it supported (from the start) the concept
of volume sets, so physical volume size was not a limiting factor, but the
logical volume characteristics were still essentially uniform.

The trouble is, files in a single file system often would like to reside on
volumes with a variety of different characteristics - in terms of access
latency, access bandwidth, replication/redundancy, etc.  Think of the
differences in access requirements between an RMS indexed file and a video
clip, for example.  A new file system could incorporate the concept of a
variety of (extendible) logical volumes, with such differing
characteristics, among which each file could be placed according to its
needs.  While one might be able to cobble up such support in the ODS-2
environment out of a sparse logical address space plus explicit placement
control based on logical block range (and suitable inheritance mechanisms in
directories for default file placement), that's not a particularly clean
implementation.