Sorry for copying almost "as is" my previous tdb backend announcement, but I 
don't really have much more to say ;-).

I've been working on metadata backend that would use tdb (meta_tdb). It's
advantages over meta_fs include:

- proper handling of "metadata" named files and dirs (Jeff Dike fixed it but 
it's still not "clean" imo, but that's the handicap of trying to implement as 
metadata files),
- compactness (no directory/files structure overhead and storing data in
binary),

Performace is yet to be tested extensively and compared to meta_fs.

There are also additions to humfs features:

- humfs stores file/dir access modes now (fixes some chmod problems, Jeff Dike 
made it part of -um2 so it's by default in UML now),
- humfs automatically initializes mount dir if only valid "superblock" file
 is present in it (humfsify for tdb is not yet ready ;),
- humfs now supports unmountig (neceseary fo meta_tdb).

Example "superblock" is attached. To try meta_fs backend with it just change
"tdb" to "shadow_fs". To mount humfs:

mount none <uml mount dir> -t humfs -o <host dir>

The patch is against UML release ver. 2.4.26-2. This release is much better 
than 2.4.26-1 when it comes to stability.

The patch is available here:

http://kem.p.lodz.pl/~peter/humfs-2.4.26-2-meta_tdb-1.patch.bz2

Humfs seems usable now (both meta_fs and meta_tdb can be used as root 
filesystem and boot), but as I tested it there are still issues, so it 
shouldn't be used in production systems. I will try to work them out 
(especially as Jeff is away for OLS, having fun I presume ;-).

Good luck and have fun testing...