[libvirt-users] backup KVM qcow2 over btrfs or zfs

Hi everybody, I'm trying to implement in the non profit organization where I work a backup strategy for our VMs. At the moment I weekly backup the machine (on sunday nights) by stopping the vm, making a snapshot, exporting the xml descriptor file and syncing these files to a remote backup server. I would really like to make daily snapshot without shutting down the vms. at the moment the vms qcow2 disks are stored over an ext4 partition but I'm planning to move them over a das with a new generation filesystem. I'm trying out zfs and btrfs looking for the best one (at the moment zfs seems to be more stable and somehow easy, flexible, while btrfs seems to have better performance on my linux box). I'm choosing one of these two FS in order to use theire instant-snapshot feature; my project will be to pause the vm, dump the memory and the xml descriptor file, make a snapshot of the subvolume storing the qcow2 file and the resuming the machine back (in I hope less than 10 seconds of downtime). what do you guys suggest me? Am I going to experience bad performance having qcow2 over btrfs? I was reading something like that but the posts were really backdated... any hints is really appreciated Thank you Francesco