libvirt users - Jan 2012 - Feasibility of iscsi storage pool

Hi,
I want to start using iscsi storage but ran into a problem. I defined a 
storage pool and this contains my first LUN which is mounted on a 
hypervisor. When I now add a second LUN on a target and do a tgt-admin 
--update --force to reload the target the i/o to the first LUN seems to 
freeze for a few seconds.
Now I'm wondering if I'm doing anything wrong? It's not really
tolerable to
have all guests using this target freeze simply because I want to add 
another LUN. The alternative is to use one LUN per target and define lots 
of targets on the server but then the whole pool configuration no longer 
makes sense as I would essentially end up with one pool per guest volume.
Does anyone have experience with this kind of setup?

Regards,
   Dennis