Messages in this thread | | | From | Peter Samuelson <> | Date | Thu, 18 Nov 1999 22:22:01 -0600 (CST) | Subject | Re: Transparent mounts (wishlist and design ideas, somewhat long) |
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[William Stearns] > I tend to like the idea that if a file exists on more than one level, > the version on the topmost media that has it wins.
Yes. This needs to be true even for mixed file types. If the bottom FS has a directory `foo/' and the top FS has a regular file `foo' and a user does chdir("foo") he gets -ENOTDIR.
> How about a simpler approach: > Reading a file would pull it off the topmost media that had it. > Writing a file would write it back to the topmost RW media - even if that > was a different media than originally held the file.
Notice the implication here: the act of mounting a FS "-o rw,trans" makes all other FSes in the stack RO (at least effectively). This being the case, I think it makes tons of sense to restrict "-o trans" to only *one* RW in the stack. This restriction helps avoid all manner of weird semantics questions and races.
> By writing to the topmost RW media, an administrator could even > stick one last RO media on the _top_ of the stack (i.e., mounted > last); any files in this media would automatically override any files > on the RW media below - although one could successfully write to > them, the OS would still use the version from the RO media on top. > An alternate way of creating immutable files, essentially.
Hmmm, I am leaning only allowing the top FS to be RW. It would IMHO be easier to implement. You do have a point in that mounting something RO over a RW FS adds flexibility.
> As above, the topmost RW media.
That's why I said all other RW mounts are effectively RO.
> It logically follows that if all media are RO, the stack is also RO.
Yeah.
> > 5. The designated partition can be different for different users.
Can o' worms. A better way to get this functionality is with loopback mounting à la AIX and others.
> > 6. Elsewhere, R/O partitions do not permit files to be modified > > in any way, so why should it be different here. > > > > 7. Why can't I boot from a CD with a hard drive mounted over it, > > and have the modifications made on the hard drive.
I vote for 7. RW means COW.
> Interesting question. Would this require some kind of > "negative dentry" that persists as long as the union is mounted? > Should that negative dentry be stored on the topmost writable media > or not?
Another can o' worms. I'd avoid this one too. A file on a RO media can be changed on the RW one (COW) but not deleted / renamed.
> For some far-in-the-future version, the concept of copy-on-read > would be marvelous too;
Yeah, far-in-the-future.
-- Peter Samuelson <sampo.creighton.edu!psamuels>
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