[Users] which file system for shared disk?

[Piotr Szubiakowski]

Hi,
we are developing an application where would be great if multiple host 
could have access to the same disk. I think that we can use features 
like shared disk or direct LUN to attach the same storage to multiple 
VM's. However to provide concurrent access to the resource, there should 
be a cluster file system used. The most popular open source cluster file 
systems are GFS2 and OCFS2. So my questions are:

1) Does anyone have share disk between VM's in oVirt? What fs did You used?
2) Is it possible to use GFS2 on VM's that are running on oVirt? Does 
anyone have run fencing mechanism with ovirt/libvirt?

Many thanks,
Piotr