vnode-backed md stops responding on copying files

I am using FreeBSD-Current (post 5.2-RELEASE) and have created a vnode backed vn
using these commands:

mdconfig -a -t vnode -s 8G -f somebackingfile -u 0 (I tried 4G and 2G, too)
bsdlabel -w md0 auto
newfs md0c
mount /dev/md0c /mnt

Then I started copying files or doing a make installworld DESTDIR=/mnt from
/usr/src and md hangs after some time copying. The md device and all processes
accessing it cannot be accessed, only a hardware reset is possible to survive,
reboot command doesn't work, too.

I was able to reproduce this system on various  system configurations (all i386,
with up to 2GB memory with AMD K6-3, Athlon or Intel P4 processors), so it
doesnt seem to be caused by my system configuration. I am using a clean FreeBSD
install.

It does not depend on mdconfig flags or if system is mounted
asynchronously/synchronously, too, tried to turn off clustering, make mount
synchronous, no help.

This problem does not occur with FreeBSD 4 (using vn driver).