On Fri, Nov 13, 2009 at 08:55:36PM +0100, Arnd Bergmann wrote: > 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. Arnd is right - only the top layer is modified. Right now, renaming a file on the lower layer requires copying up the contents of the file to the top layer, creating the new link to it, and creating a whiteout over the old name. This approach would instead leave the contents of the file on the lower layer, create a new link to it which contains the full path of the target in the lower layer, and create a whiteout over the old name. All of the modifications happen on the top layer still. > > > 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. There would be only one layer of these symlinks possible. > 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. That's certainly worth trying! -VAL -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
