Messages in this thread | | | Date | Wed, 17 Nov 1999 19:15:16 -0500 (EST) | From | Alexander Viro <> | Subject | Re: Transparent mounts |
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On Wed, 17 Nov 1999, Chad Miller wrote:
> 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.
> > 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.
You'll get some problems with readdir() on root, but that's doable. If you are going to do it - do it fast, or I'll do it myself for final procfs split.
> 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.)
Yes.
> > 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?
Uuurgh for #3 and to some degree for #2 ;-) Check how BSD folks had done it. R/O is also overkill if you are doing union-mount. Seriously, take the Daemon Book and check how it's done.
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