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    SubjectRe: Transparent mounts
    On Wed, Nov 17, 1999 at 09:26:41PM +0000, Riley Williams wrote:
    > Basically, one facility I could use is that of being able to mount two
    > or more partitions on top of each other and have all the files from
    > all of the partitions available for reading. This is what I call a
    > transparent mount.

    It sounds a lot like (BSD's?) union mount.


    > Basically, I see it initially working such that for a transparent
    > mount to succeed, BOTH of the following MUST be true:
    >
    > 1. The partition(s) being mounted over must already be mounted
    > read-only. There is therefore no problem with where to put
    > any newly created files on such a system since such can't be
    > created in the first place.

    What about appends to files on the lower, RO mount?


    > 2. There must not be any name clashes between the contents of
    > the partition(s) already mounted at that point and that of
    > the root directory of the partition being transparently
    > mounted on top of it.

    Under union-mount, the upper layer holds changes to the lower layer, so
    conflict behavior is well defined: The upper layer wins.

    (I'm not sure about a delete of a file that exists on both layers. Is
    a 'deleted' entry stored on the upper FS layer? Hmmm.)


    > I can see that verifying this second condition would be likely to be
    > time-consuming on some systems, so for an initial implementation, I
    > would suggest the following additional condition:
    >
    > 3. The total number of entries in the common directory, when
    > totalled from all partitions mounted thereon, must not be
    > such as to require more than one page of storage.
    >
    > Comments, please?

    I like the union mount, and it's very similar to yours. I'd love to
    see it in linux; I'll help, if you like.

    - chad

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