On Sat, 26 Nov 2011 21:28:51 +0530
Nirbheek Chauhan <nirbheek@g.o> wrote:

> On Sat, Nov 26, 2011 at 8:39 PM, Michał Górny <mgorny@g.o>
> wrote:
> > But in this particular case, I don't think COW is particularly
> > useful. If it works only on filesystem bounds, we could move the
> > file directly anyway.
> >
> 
> There are still a few specific cases in which CoW would indeed be
> useful. IIRC, reflinking of files works across btrfs *subvolumes*, and
> such a copy would normally be detected as a cross-device move.

For such a thing, shouldn't rename() work neat anyway?

> Another use would be an patch-merge which makes use of *ranged
> reflinks* to only CoW copy those parts of the file that were
> changed[1]. rsync has support for this, but only while appending to
> files (--append-verify --no-whole-file).

So, it'd be like:
1) CoW-dup old file,
2) patch-merge into the duped old file,
3) replace.

Am I understanding correctly?

-- 
Best regards,
Michał Górny