Some question about CephFS snapshot and related

Hi ceph community. Im Jacob and I currently surveying some solutions about cephFS offsite backup. I have read from that ceph has plan for multi-site CephFS and CephFS snapshot mirror for Octopus release from Cephalocon and issue tracker. However, i still wondering if there is any solution available right now, and it will be really helpful if you can help me answering these question How can I get diff between CephFS snapshot. I know there is rctime xattr available there, but is there any quicker solutions? Is there any way that I can specify how many OSDs of a pg required to commit when client write an object. For example, pg1 => [osd.1 osd.4 osd.7], when client write objA, he will get commit after osd.1 and osd.4 finished, and let osd.7 do the replication by recovering with ceph Thanks for spending time reading this Best regards