Hi!

NEED - provide way for SAN and hardware raid storage to do
its snapshot/copy function while the system was in-use and
get an image that could mount cleanly.  Without freeze, at
a minimum we usually needed filesystem metadata recovery
to run, worst case is completely unusable snapshits :)

freezefs() is single-level:

   ENOTSUPPOTED - by any other fs
   EOK - done
   EINPROGRESS
   EALREADY

As implemented, freezefs only ensures the metadata is
consistent so the filesystem copy can mount anywhere.

This means ONLY SOME metadata (or no metadata) is flushed and
then all metadata updates are stopped.  User/kernel writes
to already allocated file pages WILL go to a frozen disk.

That's the difference here. They do write file data, and thus avoid
mmap()-writes problem.

...and they _still_ provide auto-thaw.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html