Shared disk and snapshot

Hello,

I'm deploying a little cluster that use Red Hat GFS, so I have two VMs with one system disk (not shared) and one big shared disk.

On each VM I have a SCSI controller (paravirtual) without bus sharing (for the system disk), and a SCSI controller (paravirtual) with physical bus sharing (for the shared disk). The shared disk is INDIPENDENT.

What I want is to take a snapshot of the VM with only the system disk, but the vCenter do not allow me to take it! Is there a limitation on snapshot and bus sharing or something's wrong in my config?

Thanks,

Andrea