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SubjectRe: Overmounting a filesystem
Albert Cahalan writes:
> >> mount /dev/hda2 /home
> >> ... do things
> >> mount /dev/hda3 /home
> >
> > can I do things like that:
> >
> > mount /dev/cdrom /cdrom
> > mount /cdrom/adirectory /home/ftp/adirectory
> With Linux, I am almost sure you can not do that.
> With OSF/1 and Digital Unix, I think you can. You can mount
> one file on top of another, so directories might work too.

Unfortunately they don't. It's a special filesystem type
(FFM--"file on file mounting) and it lets you do two things.
(1) You can mount regular files, character or block special devices
on top of a file. (2) You can attach a STREAMS file descriptor to the
filesystem on top of a regular file or directory with fattach(3). You
can't mount directories. I played with this stuff with OSF/1 3.0 (or
possibly field-test T3.0) and it created a number of panics, some of
which I QAR'd and were fixed by DEC. Since I only had access to a "live"
OSF/1 system with real lusers until very recently, I haven't risked
trying out FFM again. It never seemed to quite live up to its potential.
All the neat ideas I could think of (such as "loopback" mounting of
directories and attaching ready-prepared open file descriptors to the
filesystem) never seemed to work properly. Maybe most of the problems
have been fixed by now, but I doubt it. I think the main reason for FFM
was to support SysV fattach() and it doesn't go much further.


Malcolm Beattie <>
Unix Systems Programmer
Oxford University Computing Services

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