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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