Re: unionfs

From: Horst von Brand
Date: Mon Mar 15 2004 - 14:24:10 EST

=?iso-8859-1?Q?J=F6rn?= Engel <joern@xxxxxxxxxxxxxxxxxxxx> said:
> On Mon, 15 March 2004 22:35:20 +0800, Ian Kent wrote:

> > I don't understand the requirement properly. Sorry.

> Depends on who you ask, but imo it boils down to this:
> - Use one filesystem as backing store, usually ro.
> - Have another filesystem on top for extra functionality, usually rw
> access.
> Famous example is a rw-CDROM, where writes go to hard drive and
> unchanged data is read from CDROM. But it makes sense for other
> things as well.

And what if the underlying filesystem is RW too? What should happen if you
unite several (>= 3) filesystems? What if some are RO, others RW? What do
you do if a file shows up several times, each different?

Assuming one RW on top of a RO only now: What should happen when a
file/directory is missing from the top? If the bottom one "shows through",
you can't delete anything; if it doesn't, you win nothing (because you will
have to keep a complete copy RW on top).

IIRC, this has been discussed a couple of times before, and the consensus
each time was that it isn't /that hard/ to do, it is /hard or impossible/
to find a sensible, simple semantics for this. The idea was then dropped...
Dr. Horst H. von Brand User #22616
Departamento de Informatica Fono: +56 32 654431
Universidad Tecnica Federico Santa Maria +56 32 654239
Casilla 110-V, Valparaiso, Chile Fax: +56 32 797513

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