My coworkers have found a very annoying bug. Debian’s OpenVZ kernel superior to 2.6.32-squeeze5 have a regression that make NFS exclusive file locking mechanism broken.
Therefore concurrency problems could arise when several fronts try to write into the same file. To test your setup you can do this on an NFS client:
# strace flock -x -w10 "/NFS-MOUNTPOINT/myfile" -c "sleep 10&"If it’s hang, you have a locking problem. For the moment the only solution is to downgrade your kernel. A ticket have been created on OpenVZ bugtracker.