Semantic (mountable) subvolumes in Reiser4

[Edward Shishkin <edward.shishkin@gmail.com>]

Semantic (mountable) subvolume is simply a named subtree of the semanic 
tree.
Such subtree has unusual root, which is not refered by the parent directory.
That is, parent directory doesn't contain a record about that root. So 
normally,
you can not access this subtree. In order to access a semantic 
subvolume, you
first have to "mount" it. "Mounting" a semantic subvoume creates a 
reference to
its root (specifically, the mount point refers to that root), which makes it
accessible. In other bits semantic subvolumes look like usual subtrees 
of the
semantic tree, which are always "visible".

Semantic subvolumes is not something new, they are present in some other 
file
systems (though with different names) and have many applications in
virtualization (mostly due to their "isolation").

Semantic subvolumes can be easily implemented in Reiser4. if someone cares,
then I'll provide details.

Thanks,
Edward.