unionfs limitations?

[EirikØverby]

Hi,

I just started playing with mounting ports into jails using unionfs
(mount_unionfs -b /usr/ports_jail /usr/local/jails/jail-0/usr/ports), and
many things seem to work fine.
However, when trying to install either of mysql41-server or mysql41-client,
I see the following:

[root at mpi1] /usr/ports/databases/mysql41-server# make install
===>  Installing for mysql-server-4.1.11_1
===>   mysql-server-4.1.11_1 depends on shared library: mysqlclient.14 -
found
===>   Generating temporary packing list
===>  Checking if databases/mysql41-server already installed
ln: POSIX: Operation not supported
*** Error code 1

Stop in /usr/ports/databases/mysql41-server.

Did I miss out on something, or is this not going to work? Do I need to
think in other ways?
I have stress-tested this setup pretty well over the last 24 hours, with as
many as 20 mountpoints using the same ports tree, with constant package
building in each of them. This was impossible last time I played with
unionfs, so it must have stabilized somewhat ;)

Anyone?

/Eirik