Messages in this thread | | | Date | Wed, 17 Nov 1999 18:50:37 -0500 | From | Chad Miller <> | Subject | Re: Transparent mounts |
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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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