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    SubjectRe: unionfs
    Your "what are the semantics?" arguments are mysterious to me, Horst.  I
    don't know that unionfs is a good idea, but there are trivial solutions
    to the problems you suggest. The fact that a facility can be used to
    create untenable situations does not mean that the facility is useless.

    On Mon, Mar 15, 2004 at 03:22:41PM -0400, Horst von Brand wrote:
    > =?iso-8859-1?Q?J=F6rn?= Engel <> 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?

    Only the topmost layer of a "union stack" should be RW. If you manage
    to write to an underlying FS, it is akin to writing to the block device
    underlying a normal FS -- the behavior is undefined.

    > What should happen if you unite several (>= 3) filesystems? What if
    > some are RO, others RW?

    Given that only the topmost is RW, it Just Works.

    > What do you do if a file shows up several times, each different?

    The topmost entry wins.

    > 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).

    If a directory entry is missing, the next lower layer is consulted.
    Delete is implemented with "white-out" directory entries -- a directory
    entry in the topmost FS which has special meaning, "return -ENOENT
    immediately without consulting FSs underlying me".

    > 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...

    The semantics of BSD unionfs are fairly well-defined and useful in at
    least some circumstances.


    J. S. Pendry and M. K. McKusick. Union mounts in 4.4BSD-Lite.
    In Proceedings of the USENIX Technical Conference on UNIX and Advanced
    Computing Systems, pages 25­33, December 1995.

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