Re: Fallthrus as full-length symlinks?

[Arnd Bergmann <arnd@arndb.de>]

On Friday 13 November 2009, Erez Zadok wrote:
> > The interesting thing about
> > this idea is that it could theoretically let us rename a file from the
> > low level file system to another place in the low-level file system
> > without copying the contents of the file up.  Basically, we can
> > arbitrarily swizzle the namespace of the low-level by maintaining a
> > set of symlinks above.
> 
> So now you're proposing to allow something like multiple writeable branches,
> in that you allow something other than the topmost branch to be modified.
> Moreover, it appears that what you're proposing will need to modify two or
> more branches, right?

I think you misunderstood. If the idea is that you can do a
'mv file_on_lower directory_on_higher/', you would just change the
magic symlink on the higher branch, without writing to the lower
branch, but you are able to work as if you can effectively do
operations that would otherwise require write access on the lower
branch.
 
> > Is this useful?  Is it implementable?

I think it sounds very useful and simple, but I may also have missed something.
One issue might be that with a lot of file systems stacked on top of
each other, you could run out of stack space recursing through the
symlinks, but that's something that may already be the case, or that
can be worked around in other ways.

Another idea that I first had when reading the suggestion was to use a
symlink to self (ln -s x x) as the encoding for a fallthrough. It does
not allow renames like what you really describe, but it has another advantage
in that it does not require extensions to the upper file system layout
while not conflicting with any use case I can see.

	Arnd <><