[Users] which file system for shared disk?

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