Re: unionfs

[Jamie Lokier <jamie@shareable.org>]

Jörn Engel wrote:
> And version control is something I actually want to be done inside the
> kernel, at least to some degree.  People already use kernel support,
> although it sucks (cp -lr anyone?).  Looks like the alternatives suck
> even more, so your point is void.

Fwiw, I much prefer your COW hard links to something where I have to
mount a new filesystem every time I "copy" a tree, and have to redo
those mounts each time I reboot, have a big ugly mess in "df" output,
what "du" get confused, and "rsync" has no hope of dealing with them
sensibly.

I also don't mind if copying isn't implemented in the kernel.  I'm ok
with programs reporting an error that they couldn't write to a file
because it was linked readonly.  At least that removes the danger of
accidental overwriting, and I can either fix it by hand or use an
LD_PRELOAD library which detects that error code from open() and
copies the file.

Even if vi and Emacs, which make it temptingly easy to ignore normal
read-only protection, were changed to be aware of and bypass the
read-only link attribute, they'd do the right thing: the attribute
expresses the _intent_ that removing it should always be done by
copying the file, whereas with hard links that intent isn't clear.
(Emacs has backup-by-copying-when-linked, but that isn't too helpful
because sometimes you want writing to a linked file to change both places).

So my vote is for the very simple COW hard link attribute, and leave
the rest to userspace.

Thanks!
-- Jamie