I did a little more testing.

root@virt:~# uname -r
3.13.0-32-generic


Actually the hanging seams to happen only when the overlayfs parts are on the 
logical volume being the snapshot source.
Let's say /opt is a logical volume in the volume group virt.

Case 1 (hanging):
mkdir /opt/test/
cd /opt/test/
mkdir upper lower file_system
touch lower/bla.dat

mount -t overlayfs -o lowerdir=/opt/test/lower,upperdir=/opt/test/upper
overlayfs /opt/test/file_system

lvcreate -s -L 5G /dev/virt/opt -n vm_backup

-> Hang (see previous posts)

Case 2 (not hanging):
mkdir /opt/test/
lvcreate -n test -L10G virt
mkfs.xfs /dev/virt/test
mount /dev/virt/test /opt/test
cd /opt/test/
mkdir upper lower file_system
touch lower/bla.dat
mount -t overlayfs -o lowerdir=/opt/test/lower,upperdir=/opt/test/upper 
overlayfs /opt/test/file_system
lvcreate -s -L 5G /dev/virt/opt -n vm_backup 

Snapshot created even through the overlayfs is in use (but  on a
different volume).

So the work around for the moment is to use two types of logical volumes :
1 one type for use with overlayfs where one has to make sure that no one ever 
takes a snapshot of (at least until this bug is fixed.)
2. one type where overlayfs never gets used but where snapshots can be taken 
from.

May be this helps someone running into this issue.

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https://bugs.launchpad.net/bugs/1328595

Title:
  lvcreate -s hangs when overlayfs is in use

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